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_squash_branch.py

@@ -0,0 +1,20 @@
+#!/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.
+
+import argparse
+import sys
+
+from git_common import squash_current_branch
+
+def main(args):
+  parser = argparse.ArgumentParser()
+  parser.add_argument(
+      '-m', '--message', metavar='<msg>', default='git squash commit.',
+      help='Use the given <msg> as the first line of the commit message.')
+  opts = parser.parse_args(args)
+  squash_current_branch(opts.message)
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv))