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_mark_merge_base.py

@@ -0,0 +1,68 @@
+#!/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.
+
+"""
+Explicitly set/remove/print the merge-base for the current branch.
+
+This manually set merge base will be a stand-in for `git merge-base` for the
+purposes of the chromium depot_tools git extensions. Passing no arguments will
+just print the effective merge base for the current branch.
+"""
+
+import argparse
+import sys
+
+from subprocess2 import CalledProcessError
+
+from git_common import remove_merge_base, manual_merge_base, current_branch
+from git_common import get_or_create_merge_base, hash_one
+
+
+def main(argv):
+  parser = argparse.ArgumentParser(
+    description=__doc__.strip().splitlines()[0],
+    epilog=' '.join(__doc__.strip().splitlines()[1:]))
+  g = parser.add_mutually_exclusive_group()
+  g.add_argument(
+    'merge_base', nargs='?',
+    help='The new hash to use as the merge base for the current branch'
+  )
+  g.add_argument('--delete', '-d', action='store_true',
+                 help='Remove the set mark.')
+  opts = parser.parse_args(argv)
+
+  cur = current_branch()
+
+  if opts.delete:
+    try:
+      remove_merge_base(cur)
+    except CalledProcessError:
+      print "No merge base currently exists for %s." % cur
+    return 0
+
+  if opts.merge_base:
+    try:
+      opts.merge_base = hash_one(opts.merge_base)
+    except CalledProcessError:
+      print >> sys.stderr, (
+          'fatal: could not resolve %s as a commit' % (opts.merge_base)
+      )
+      return 1
+
+    manual_merge_base(cur, opts.merge_base)
+
+  ret = 0
+  actual = get_or_create_merge_base(cur)
+  if opts.merge_base and opts.merge_base != actual:
+    ret = 1
+    print "Invalid merge_base %s" % opts.merge_base
+
+  print "merge_base(%s): %s" % (cur, actual)
+
+  return ret
+
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv[1:]))