Git repo import fails if gpgsig is encountered

Git repo import fails with "bzrlib.plugins.git.errors.UnknownCommitExtra" if any of the git commits contain a gpgsig

http://launchpadlibrarian.net/347931559/nick-nickdowell-amsynth-github.log

2017-12-03 14:49:49 INFO Starting job.
2017-12-03 14:49:49 INFO Getting exising bzr branch from central store.
2017-12-03 14:49:50 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2017-12-03 14:49:51 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2017-12-03 14:49:51 INFO 287 bytes transferred
2017-12-03 14:49:52 INFO Importing branch.
2017-12-03 14:49:52 INFO Counting objects: 6688, done. 0
2017-12-03 14:49:53 INFO finding revisions to fetch:generating index 0/6688
2017-12-03 14:49:54 INFO finding revisions to fetch:generating index 0/6688
2017-12-03 14:49:55 INFO finding revisions to fetch 1/8
2017-12-03 14:49:55 INFO
Traceback (most recent call last):
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/scripts/code-import-worker.py", line 106, in <module>
    sys.exit(script.main())
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/scripts/code-import-worker.py", line 101, in main
    return import_worker.run()
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/lib/lp/codehosting/codeimport/worker.py", line 617, in run
    return self._doImport()
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/lib/lp/codehosting/codeimport/worker.py", line 820, in _doImport
    inter_branch.fetch(limit=revision_limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/bzrplugins/git/branch.py", line 722, in fetch
    self.fetch_objects(stop_revision, fetch_tags=fetch_tags, limit=limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/bzrplugins/git/branch.py", line 745, in fetch_objects
    determine_wants, self.source.mapping, limit=limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/bzrplugins/git/fetch.py", line 718, in fetch_objects
    limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/bzrplugins/git/fetch.py", line 484, in import_git_objects
    mapping.revision_id_foreign_to_bzr)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18511/bzrplugins/git/mapping.py", line 334, in import_commit
    raise UnknownCommitExtra(commit, [item[0] for item in commit.extra])
bzrlib.plugins.git.errors.UnknownCommitExtra: Unknown extra fields in <Commit 2c614240a51587e4a70365468ac0fb62ac2b37f4>: ['gpgsig'].
Import failed:
Traceback (most recent call last):
Failure: twisted.internet.error.ProcessTerminated: A process has ended with a probable error condition: process ended with exit code 1.