Some github imports fail due to gpgsig error

[Nicolas DERIVE]

When requesting some bazaar imports in Launchpad, we got the following error, preventing import jobs to perform:

2016-09-27 15:07:52 INFO Starting job.
2016-09-27 15:07:52 INFO Getting exising bzr branch from central store.
2016-09-27 15:07:52 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2016-09-27 15:07:52 INFO 35 bytes transferred
2016-09-27 15:07:58 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2016-09-27 15:08:00 INFO Importing branch.
2016-09-27 15:08:03 INFO Counting objects: 851, done. 0
2016-09-27 15:08:03 INFO finding revisions to fetch:generating index 0/851
2016-09-27 15:08:04 INFO finding revisions to fetch:generating index 0/851
2016-09-27 15:08:04 INFO finding revisions to fetch 1
2016-09-27 15:08:04 INFO
Traceback (most recent call last):
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/scripts/code-import-worker.py", line 96, in <module>
    sys.exit(script.main())
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/scripts/code-import-worker.py", line 91, in main
    return import_worker.run()
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/lib/lp/codehosting/codeimport/worker.py", line 583, in run
    return self._doImport()
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/lib/lp/codehosting/codeimport/worker.py", line 737, in _doImport
    inter_branch.fetch(limit=revision_limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/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-18204/bzrplugins/git/branch.py", line 745, in fetch_objects
    determine_wants, self.source.mapping, limit=limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/bzrplugins/git/fetch.py", line 718, in fetch_objects
    limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/bzrplugins/git/fetch.py", line 484, in import_git_objects
    mapping.revision_id_foreign_to_bzr)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18204/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 0c5a72d5782575794feba5c97089df8a10441383>: ['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.