rewrite: add conflict labels for squashed commits

An example of squashing `ysrnknol` into `rtsqusxu`:

```
<<<<<<< conflict 1 of 1
+++++++ squash destination (rtsqusxu 2768b0b9)
updated in destination
%%%%%%% squashed commit (ysrnknol 7a20f389) compared with parents of ysrnknol 7a20f389
-base
+squashed
>>>>>>> conflict 1 of 1 ends
```

Author: scott2000

cli/tests/test_squash_command.rs

@@ -872,26 +872,26 @@ fn test_squash_from_multiple() {
 
     // Squash a few commits sideways
     let output = work_dir.run_jj(["squash", "--from=b", "--from=c", "--into=d"]);
-    insta::assert_snapshot!(output, @r###"
+    insta::assert_snapshot!(output, @r"
     ------- stderr -------
     Rebased 2 descendant commits
-    Working copy  (@) now at: kpqxywon 0b695306 f | (no description set)
-    Parent commit (@-)      : yostqsxw ff064d52 e | (no description set)
+    Working copy  (@) now at: kpqxywon 5c538630 f | (no description set)
+    Parent commit (@-)      : yostqsxw fd2940e4 e | (no description set)
     New conflicts appeared in 1 commits:
-      yqosqzyt 61130da4 d | (conflict) (no description set)
+      yqosqzyt c31cb991 d | (conflict) (no description set)
     Hint: To resolve the conflicts, start by creating a commit on top of
     the conflicted commit:
       jj new yqosqzyt
     Then use `jj resolve`, or edit the conflict markers in the file directly.
     Once the conflicts are resolved, you can inspect the result with `jj diff`.
     Then run `jj squash` to move the resolution into the conflicted commit.
     [EOF]
-    "###);
+    ");
     insta::assert_snapshot!(get_log_output(&work_dir), @r"
-    @  0b6953066ee0 f
-    ○    ff064d529578 e
+    @  5c5386309539 f
+    ○    fd2940e4f9c1 e
     ├─╮
-    × │  61130da4e714 d
+    × │  c31cb991c9be d
     ├─╯
     ○  e88768e65e67 a b c
     ◆  000000000000 (empty)
@@ -901,13 +901,13 @@ fn test_squash_from_multiple() {
     let output = work_dir.run_jj(["file", "show", "-r=d", "file"]);
     insta::assert_snapshot!(output, @r"
     <<<<<<< conflict 1 of 1
-    %%%%%%% side #1 compared with base #1
+    %%%%%%% squash destination (yqosqzyt 8acbb715) compared with parents of kkmpptxz fed4d1a2
     -a
     +d
-    %%%%%%% side #2 compared with base #2
+    %%%%%%% squashed commit (kkmpptxz fed4d1a2) compared with parents of mzvwutvl d7e94ec7
     -a
     +b
-    +++++++ side #3
+    +++++++ squashed commit (mzvwutvl d7e94ec7)
     c
     >>>>>>> conflict 1 of 1 ends
     [EOF]
@@ -1016,29 +1016,29 @@ fn test_squash_from_multiple_partial() {
 
     // Partially squash a few commits sideways
     let output = work_dir.run_jj(["squash", "--from=b|c", "--into=d", "file1"]);
-    insta::assert_snapshot!(output, @r###"
+    insta::assert_snapshot!(output, @r"
     ------- stderr -------
     Rebased 2 descendant commits
-    Working copy  (@) now at: kpqxywon a724910c f | (no description set)
-    Parent commit (@-)      : yostqsxw 1bc405e1 e | (no description set)
+    Working copy  (@) now at: kpqxywon 5f09dbd4 f | (no description set)
+    Parent commit (@-)      : yostqsxw 602ec1ab e | (no description set)
     New conflicts appeared in 1 commits:
-      yqosqzyt 7ddfe685 d | (conflict) (no description set)
+      yqosqzyt ed2751a9 d | (conflict) (no description set)
     Hint: To resolve the conflicts, start by creating a commit on top of
     the conflicted commit:
       jj new yqosqzyt
     Then use `jj resolve`, or edit the conflict markers in the file directly.
     Once the conflicts are resolved, you can inspect the result with `jj diff`.
     Then run `jj squash` to move the resolution into the conflicted commit.
     [EOF]
-    "###);
+    ");
     insta::assert_snapshot!(get_log_output(&work_dir), @r"
-    @  a724910cd361 f
-    ○      1bc405e12b68 e
+    @  5f09dbd46785 f
+    ○      602ec1ab8c78 e
     ├─┬─╮
     │ │ ○  e9db15b956c4 b
     │ ○ │  83cbe51db94d c
     │ ├─╯
-    × │  7ddfe6857387 d
+    × │  ed2751a98f74 d
     ├─╯
     ○  64ea60be8d77 a
     ◆  000000000000 (empty)
@@ -1059,13 +1059,13 @@ fn test_squash_from_multiple_partial() {
     let output = work_dir.run_jj(["file", "show", "-r=d", "file1"]);
     insta::assert_snapshot!(output, @r"
     <<<<<<< conflict 1 of 1
-    %%%%%%% side #1 compared with base #1
+    %%%%%%% squash destination (yqosqzyt f6812ff8) compared with parents of kkmpptxz f2c9709f
     -a
     +d
-    %%%%%%% side #2 compared with base #2
+    %%%%%%% squashed commit (selected changes from kkmpptxz f2c9709f) compared with parents of mzvwutvl aa908686
     -a
     +b
-    +++++++ side #3
+    +++++++ squashed commit (selected changes from mzvwutvl aa908686)
     c
     >>>>>>> conflict 1 of 1 ends
     [EOF]

lib/src/merge.rs

@@ -22,6 +22,7 @@ use std::fmt::Formatter;
 use std::fmt::Write as _;
 use std::future::Future;
 use std::hash::Hash;
+use std::iter;
 use std::iter::zip;
 use std::slice;
 use std::sync::Arc;
@@ -210,6 +211,15 @@ impl<T> Merge<T> {
         Self { values }
     }
 
+    /// Creates a `Merge` from a first side and a series of diffs to apply to
+    /// that side.
+    pub fn from_diffs(first_side: T, diffs: impl IntoIterator<Item = Diff<T>>) -> Self {
+        let values = iter::once(first_side)
+            .chain(diffs.into_iter().flat_map(|diff| [diff.before, diff.after]))
+            .collect();
+        Self { values }
+    }
+
     /// Creates a `Merge` with a single resolved value.
     pub const fn resolved(value: T) -> Self {
         Self {

lib/src/rewrite.rs

@@ -39,6 +39,7 @@ use crate::index::IndexResult;
 use crate::iter_util::fallible_any;
 use crate::matchers::Matcher;
 use crate::matchers::Visit;
+use crate::merge::Diff;
 use crate::merge::Merge;
 use crate::merged_tree::MergedTree;
 use crate::merged_tree::MergedTreeBuilder;
@@ -1155,6 +1156,20 @@ impl CommitWithSelection {
     pub fn is_empty_selection(&self) -> bool {
         self.selected_tree.tree_ids() == self.parent_tree.tree_ids()
     }
+
+    /// Returns conflict labels for the diff represented by this selection.
+    pub fn conflict_labels(&self) -> BackendResult<Diff<String>> {
+        let commit_label = self.commit.conflict_label();
+        let parent_label = format!("parents of {commit_label}");
+        if self.is_full_selection() {
+            Ok(Diff::new(parent_label, commit_label))
+        } else {
+            Ok(Diff::new(
+                parent_label,
+                format!("selected changes from {commit_label}"),
+            ))
+        }
+    }
 }
 
 /// Resulting commit builder and stats to be returned by [`squash_commits()`].
@@ -1177,6 +1192,7 @@ pub fn squash_commits<'repo>(
 ) -> BackendResult<Option<SquashedCommit<'repo>>> {
     struct SourceCommit<'a> {
         commit: &'a CommitWithSelection,
+        labels: Diff<String>,
         abandon: bool,
     }
     let mut source_commits = vec![];
@@ -1193,6 +1209,7 @@ pub fn squash_commits<'repo>(
         // squash -r`)? The source tree will be unchanged in that case.
         source_commits.push(SourceCommit {
             commit: source,
+            labels: source.conflict_labels()?,
             abandon,
         });
     }
@@ -1209,12 +1226,18 @@ pub fn squash_commits<'repo>(
         } else {
             let source_tree = source.commit.commit.tree();
             // Apply the reverse of the selected changes onto the source
-            let new_source_tree = source_tree
-                .merge_unlabeled(
+            let new_source_tree = MergedTree::merge(Merge::from_vec(vec![
+                (source_tree, source.commit.commit.conflict_label()),
+                (
                     source.commit.selected_tree.clone(),
+                    source.labels.after.clone(),
+                ),
+                (
                     source.commit.parent_tree.clone(),
-                )
-                .block_on()?;
+                    source.labels.before.clone(),
+                ),
+            ]))
+            .block_on()?;
             repo.rewrite_commit(&source.commit.commit)
                 .set_tree(new_source_tree)
                 .write()?;
@@ -1244,22 +1267,29 @@ pub fn squash_commits<'repo>(
             };
         })?;
     }
-    // Apply the selected changes onto the destination
-    let mut destination_tree = rewritten_destination.tree();
-    for source in &source_commits {
-        destination_tree = destination_tree
-            .merge_unlabeled(
-                source.commit.parent_tree.clone(),
-                source.commit.selected_tree.clone(),
-            )
-            .block_on()?;
-    }
     let mut predecessors = vec![destination.id().clone()];
     predecessors.extend(
         source_commits
             .iter()
             .map(|source| source.commit.commit.id().clone()),
     );
+    // Apply the selected changes onto the destination
+    let destination_tree = MergedTree::merge(Merge::from_diffs(
+        (
+            rewritten_destination.tree(),
+            format!("squash destination ({})", destination.conflict_label()),
+        ),
+        source_commits.into_iter().map(|source| {
+            Diff::new(
+                (source.commit.parent_tree.clone(), source.labels.before),
+                (
+                    source.commit.selected_tree.clone(),
+                    format!("squashed commit ({})", source.labels.after),
+                ),
+            )
+        }),
+    ))
+    .block_on()?;
 
     let commit_builder = repo
         .rewrite_commit(&rewritten_destination)