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