Fail with user friendly error when no new parent branch is specified.

Before this change, running
$ git reparent-branch
produced stacktrace and
"TypeError: sequence item 4: expected string, NoneType found"

R=iannucci@chromium.org
TEST=manual + unittest + coverage.
BUG=

Review URL: https://codereview.chromium.org/1280633003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296235 0039d316-1c4b-4281-b951-d872f2087c98

git_reparent_branch.py

@@ -36,6 +36,8 @@ def main(args):
   elif opts.lkgr:
     new_parent = 'lkgr'
   else:
+    if not opts.new_parent:
+      parser.error('Must specify new parent somehow')
     new_parent = opts.new_parent
   cur_parent = upstream(branch)
 

tests/git_rebase_update_test.py

@@ -152,6 +152,8 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
     self.assertEqual(self.repo.git('status', '--porcelain').stdout, '?? bob\n')
 
     self.repo.git('checkout', 'origin/master')
+    _, err = self.repo.capture_stdio(self.rp.main, [])
+    self.assertIn('Must specify new parent somehow', err)
     _, err = self.repo.capture_stdio(self.rp.main, ['foobar'])
     self.assertIn('Must be on the branch', err)