dir_traversal: Prioritize .exclude rules over .exclude_from rules

Ensure rules read from files (i.e. passed to .exclude_from() are
inserted _before_ any rules parsed from patterns passed to .exclude().

Later rules override earlier rules, so this allows a user to override a
pattern from a file with a pattern passed directly on the command line.

For example, we want "--exclude !foo/bar" on the command line to always
override a "foo/*" pattern read from a file.

fawltydeps/dir_traversal.py

@@ -173,7 +173,9 @@ def exclude_from(self, file_with_exclude_patterns: Path) -> None:
         See .exclude() for details about how each gitignore pattern is used.
         """
         logger.debug(f"Reading exclude patterns from {file_with_exclude_patterns}...")
-        self.exclude_rules.extend(parse_gitignore(file_with_exclude_patterns))
+        self.exclude_rules = (
+            list(parse_gitignore(file_with_exclude_patterns)) + self.exclude_rules
+        )
 
     def is_excluded(self, path: Path, is_dir: bool) -> bool:
         """Check if given path is excluded by any of our exclude rules."""

tests/test_dir_traversal.py

@@ -1005,6 +1005,25 @@ def on_windows(msg: str) -> Optional[str]:
             ),
         ],
     ),
+    #
+    # Testing combination of .exclude() and .exclude_from() patterns
+    #
+    DirectoryTraversalVector(
+        "exclude_patterns_take_precedence_over_exclude_from_patterns",
+        given=[
+            File(".gitignore", "foo/*"),  # exclude everything inside foo/
+            File("foo/bar"),  # NOT excluded due to exclude_pattern below
+            File("foo/baz"),  # excluded
+        ],
+        exclude_patterns=[
+            ExcludePattern("!foo/bar"),  # overrides "foo/*" in .gitignore
+        ],
+        exclude_from=[".gitignore"],
+        expect=[
+            ExpectedTraverseStep(".", subdirs=["foo"], files=[".gitignore"]),
+            ExpectedTraverseStep("foo", files=["bar"], excluded_files=["baz"]),
+        ],
+    ),
 ]