Add a '--inject-current' option to 'git new-branch'

This option creates a new branch that sets itself as the upstream for
the current branch and adopts the current branch's upstream as its own.

What does it do?
----------------

Imagine we have the following set of branches (as `git map-branches -vv`
would show it):

  origin/master  00001111  [ ahead 1 ]
    foo          00002222  [ ahead 1 ]
      baz *      00003333  [ ahead 1 ]

'baz' is the current branch. If one were to issue the following command:

  $ git new-branch --inject-current bar

... then the branch layout will now look like this:

  origin/master  00001111  [ ahead 1 ]
    foo          00002222  [ ahead 1 ]
      bar *      00002222
        baz      00003333  [ ahead 1 ]

Why would you need this?
------------------------

When working on a single change or a sequence of changes organized into
dependent branches, this option lets you quickly create a new branch
with the correct upstreams so that you can peel off smaller cleanups and
potentially unrelated changes out of your main feature branch into
separate branches. These can then be uploaded as dependent CLs.

R=petermayo@chromium.org,iannucci@chromium.org

Change-Id: Id912f8e5c17e267fc52d74bdfac7bbcf87a50908
Reviewed-on: https://chromium-review.googlesource.com/987529
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

git_new_branch.py

@@ -2,7 +2,6 @@
 # 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.
-
 """
 Create new branch tracking origin/master by default.
 """
@@ -13,7 +12,7 @@ import sys
 import subprocess2
 
 from git_common import run, root, set_config, get_or_create_merge_base, tags
-from git_common import hash_one
+from git_common import hash_one, upstream, set_branch_config, current_branch
 
 
 def main(args):
@@ -28,13 +27,26 @@ def main(args):
                  help='set upstream branch to current branch.')
   g.add_argument('--upstream', metavar='REF', default=root(),
                  help='upstream branch (or tag) to track.')
+  g.add_argument('--inject-current', '--inject_current',
+                 action='store_true',
+                 help='new branch adopts current branch\'s upstream,' +
+                 ' and new branch becomes current branch\'s upstream.')
   g.add_argument('--lkgr', action='store_const', const='lkgr', dest='upstream',
                  help='set basis ref for new branch to lkgr.')
 
   opts = parser.parse_args(args)
 
   try:
-    if opts.upstream_current:
+    if opts.inject_current:
+      below = current_branch()
+      if below is None:
+        raise Exception('no current branch')
+      above = upstream(below)
+      if above is None:
+        raise Exception('branch %s has no upstream' % (below))
+      run('checkout', '--track', above, '-b', opts.branch_name)
+      run('branch', '--set-upstream-to', opts.branch_name, below)
+    elif opts.upstream_current:
       run('checkout', '--track', '-b', opts.branch_name)
     else:
       if opts.upstream in tags():