Add docs to Committing process

Author: ctapobep

pseudogit/Committing.groovy

@@ -18,12 +18,19 @@ Tree root = new Tree([
         'dir'  : d1,
         'dir2' : d2,
         'c.txt': b1])
-Commit c1 = new Commit(root)
+Commit c1 = new Commit('Initial commit', root)
 /**
- * This is an empty commit since it references the same tree, so it doesn't change the tree structure
+ * This is an empty commit since it references the same tree, so it doesn't change the tree structure:
+ * {@code git commit --allow-empty}
+ */
+Commit c2 = new Commit('Empty commit since it references the same tree as its parent', root, c1)
+/**
+ * New files were added:
+ *
+ * echo 'Hello' > dir/a.txt
+ * echo 'blah' > dir/d.txt
+ * git add -u
  */
-Commit c2 = new Commit(root, c1) // same tree, so it's an empty commit
-
 d1 = new Tree([
         'a.txt': b1,
         'd.txt': new Blob('blah'.bytes)])
@@ -38,8 +45,13 @@ d1 = new Tree([
  *     c.txt  // new content!
  * </pre>
  */
-Commit c3 = new Commit(new Tree([// new root
-        'dir': d1,// new d1 with additional file
-        'dir2': d2,
-        'c.txt': new Blob("blah2".bytes)/*file content changed*/],
-), c2)
+root = new Tree([
+        'dir'  : d1,// new d1 with additional file
+        'dir2' : d2,
+        'c.txt': new Blob("blah2".bytes)/*additionally changed the file content*/],
+)
+/**
+ * Now new commit references new Tree and prev commit becomes its parent:
+ * git commit -m '...'
+ */
+Commit c3 = new Commit('New commit references new root Tree because it in turn references new sub-Tree', root, c2)

pseudogit/GitLog.groovy

@@ -1,7 +1,7 @@
 import objects.*
 
 Tree t = new Tree()
-Commit commit = new Commit(t, new Commit(t, new Commit(t, new Commit())))
+Commit commit = new Commit('1st', t, new Commit('2nd', t, new Commit('3d', t, new Commit('4th', t))))
 
 for(Commit c = commit; c.parent != null; c = c.parent)
     println c

pseudogit/objects/Commit.groovy

@@ -1,10 +1,12 @@
 package objects
 
 class Commit {
+    String message
     Tree root
     Commit parent
 
-    Commit(Tree root, Commit parent = null) {
+    Commit(String message, Tree root, Commit parent = null) {
+        this.message = message
         this.root = root
         this.parent = parent
     }