"IndexError: string index out of range" in version.from_pip_string

[reported on behalf or R. Nortman (r-nortman)]

Changelog parsing changed between 1.8.1 and 1.9.0 with https://review.openstack.org/263748 "Split changelog on nulls instead of (" and now some tag patterns cause the following traceback:

  [pbr] Writing ChangeLog
  [pbr] Generating ChangeLog
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/tmp/pip-f3fao6-build/setup.py", line 29, in <module>
      pbr=True)
    File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
      dist.run_commands()
    File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
      self.run_command(cmd)
    File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
      cmd_obj.run()
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/wheel/bdist_wheel.py", line 211, in run
      self.run_command('install')
    File "/usr/lib/python2.7/distutils/cmd.py", line 326, in run_command
      self.distribution.run_command(command)
    File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
      cmd_obj.run()
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/pbr/packaging.py", line 195, in run
      _from_git(self.distribution)
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/pbr/packaging.py", line 485, in _from_git
      git.write_git_changelog(option_dict=option_dict, changelog=changelog)
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/pbr/git.py", line 261, in write_git_changelog
      for release, content in changelog:
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/pbr/git.py", line 156, in _iter_changelog
      for hash, tags, msg in changelog:
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/pbr/git.py", line 230, in _iter_log_inner
      if _is_valid_version(candidate):
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/pbr/git.py", line 192, in _is_valid_version
      version.SemanticVersion.from_pip_string(candidate)
    File "/home/rnortman/.virtualenvs/testpbr/local/lib/python2.7/site-packages/pbr/version.py", line 200, in from_pip_string
      if remainder and (remainder[0][0] == '0' or
  IndexError: string index out of range