Add a family of git-commands to assist with the management of multiple CLs/branches.

git-rebase-update - ensure all branches are up to date
git-new-branch - create branches
git-rename-branch - rename a branch while preserving parentage relationships
git-reparent-branch - change the parent of a branch, including rebasing it correctly onto that new parent.
git-squash-branch - collapse a branch into a single commit
git-upstream-diff - show the diff between the current branch and it's upstream branch
git-mark-merge-base - explicitly set what you want the above tools to consider the merge-base for the current branch.

R=agable@chromium.org, hinoka@chromium.org, stip@chromium.org, szager@chromium.org
BUG=261738

Review URL: https://codereview.chromium.org/184253003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@259520 0039d316-1c4b-4281-b951-d872f2087c98

git_reparent_branch.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Change the upstream of the current branch."""
+
+import argparse
+import sys
+
+import subprocess2
+
+from git_common import upstream, current_branch, run, tags, set_branch_config
+from git_common import get_or_create_merge_base, root
+
+import git_rebase_update
+
+def main(args):
+  root_ref = root()
+
+  parser = argparse.ArgumentParser()
+  g = parser.add_mutually_exclusive_group()
+  g.add_argument('new_parent', nargs='?',
+                 help='New parent branch (or tag) to reparent to.')
+  g.add_argument('--root', action='store_true',
+                 help='Reparent to the configured root branch (%s).' % root_ref)
+  g.add_argument('--lkgr', action='store_true',
+                 help='Reparent to the lkgr tag.')
+  opts = parser.parse_args(args)
+
+  # TODO(iannucci): Allow specification of the branch-to-reparent
+
+  branch = current_branch()
+  if opts.root:
+    new_parent = root_ref
+  elif opts.lkgr:
+    new_parent = 'lkgr'
+  else:
+    new_parent = opts.new_parent
+  cur_parent = upstream(branch)
+
+  if branch == 'HEAD' or not branch:
+    parser.error('Must be on the branch you want to reparent')
+  if new_parent == cur_parent:
+    parser.error('Cannot reparent a branch to its existing parent')
+
+  get_or_create_merge_base(branch, cur_parent)
+
+  all_tags = tags()
+  if cur_parent in all_tags:
+    cur_parent += ' [tag]'
+
+  try:
+    run('show-ref', new_parent)
+  except subprocess2.CalledProcessError:
+    print >> sys.stderr, 'fatal: invalid reference: %s' % new_parent
+    return 1
+
+  if new_parent in all_tags:
+    print ("Reparenting %s to track %s [tag] (was %s)"
+           % (branch, new_parent, cur_parent))
+    set_branch_config(branch, 'remote', '.')
+    set_branch_config(branch, 'merge', new_parent)
+  else:
+    print ("Reparenting %s to track %s (was %s)"
+           % (branch, new_parent, cur_parent))
+    run('branch', '--set-upstream-to', new_parent, branch)
+
+  # TODO(iannucci): ONLY rebase-update the branch which moved (and dependants)
+  return git_rebase_update.main(['--no_fetch'])
+
+
+if __name__ == '__main__':  # pragma: no cover
+  sys.exit(main(sys.argv[1:]))