macOS 10.12

What steps will reproduce the problem?
(1) Run `git-map-branches --show-subject` in a Chrome checkout.

What is the expected result?
No crash.

What happens instead?
Traceback (most recent call last):
  File "/Users/lgarron/Code/depot_tools/git_map_branches.py", line 329, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/Users/lgarron/Code/depot_tools/git_map_branches.py", line 323, in main
    mapper.start()
  File "/Users/lgarron/Code/depot_tools/git_map_branches.py", line 171, in start
    self.__append_branch(root)
  File "/Users/lgarron/Code/depot_tools/git_map_branches.py", line 272, in __append_branch
    line.append(run('log', '-n1', '--format=%s', branch, '--'))
  File "/Users/lgarron/Code/depot_tools/git_common.py", line 688, in run
    return run_with_stderr(*cmd, **kwargs)[0]
  File "/Users/lgarron/Code/depot_tools/git_common.py", line 757, in run_with_stderr
    raise subprocess2.CalledProcessError(retcode, cmd, os.getcwd(), ret, err)
subprocess2.CalledProcessError: Command 'git -c color.ui=never log -n1 --format=%s  --' returned non-zero exit status 128 in /Users/lgarron/chromium/src
fatal: bad revision ''