reparent: add a --commit option

git-imerge

@@ -3628,19 +3628,63 @@ def cmd_diagram(parser, options):
         )
 
 
+def reparent_recursively(git, start_commit, parents, end_commit):
+    """Change the parents of start_commit and its descendants.
+
+    Change start_commit to have the specified parents, and reparent
+    all commits on the ancestry path between start_commit and
+    end_commit accordingly. Return the replacement end_commit.
+    start_commit, parents, and end_commit must all be resolved OIDs.
+
+    """
+
+    # A map {old_oid : new_oid} keeping track of which replacements
+    # have to be made:
+    replacements = {}
+
+    # Reparent start_commit:
+    replacements[start_commit] = git.reparent(start_commit, parents)
+
+    for (commit, parents) in git.rev_list_with_parents(
+            '--ancestry-path', '--topo-order', '--reverse',
+            '%s..%s' % (start_commit, end_commit)
+            ):
+        parents = [replacements.get(p, p) for p in parents]
+        replacements[commit] = git.reparent(commit, parents)
+
+    try:
+        return replacements[end_commit]
+    except KeyError:
+        raise ValueError(
+            "%s is not an ancestor of %s" % (start_commit, end_commit),
+        )
+
+
 def cmd_reparent(parser, options):
     git = GitRepository()
     try:
-        commit_sha1 = git.get_commit_sha1('HEAD')
+        commit = git.get_commit_sha1(options.commit)
+    except ValueError:
+        sys.exit('%s is not a valid commit', options.commit)
+
+    try:
+        head = git.get_commit_sha1('HEAD')
     except ValueError:
         sys.exit('HEAD is not a valid commit')
 
     try:
-        parent_sha1s = [git.get_commit_sha1(p) for p in options.parents]
+        parents = [git.get_commit_sha1(p) for p in options.parents]
+    except ValueError as e:
+        sys.exit(e.message)
+
+    sys.stderr.write('Reparenting %s..HEAD\n' % (options.commit,))
+
+    try:
+        new_head = reparent_recursively(git, commit, parents, head)
     except ValueError as e:
         sys.exit(e.message)
 
-    sys.stdout.write('%s\n' % (git.reparent(commit_sha1, parent_sha1s),))
+    sys.stdout.write('%s\n' % (new_head,))
 
 
 def main(args):
@@ -3927,10 +3971,24 @@ def main(args):
 
     subparser = subparsers.add_parser(
         'reparent',
-        help='change the parents of the HEAD commit',
+        help=(
+            'change the parents of the specified commit and propagate the '
+            'change to HEAD'
+            ),
         )
     subparser.add_argument(
-        'parents', nargs='*', help='[PARENT...]',
+        '--commit', metavar='COMMIT', default='HEAD',
+        help=(
+            'target commit to reparent. Create a new commit identical to '
+            'this one, but having the specified parents. Then create '
+            'new versions of all descendants of this commit all the way to '
+            'HEAD, incorporating the modified commit. Output the SHA-1 of '
+            'the replacement HEAD commit.'
+        ),
+    )
+    subparser.add_argument(
+        'parents', nargs='*', metavar='PARENT',
+        help='a list of commits',
         )
 
     options = parser.parse_args(args)