external: trigger git commit-msg hooks on import

external: trigger git commit-msg hooks on import

When we import commits, the new commit does not have a Change-Id in
the commit message. Usually a git commit-msg hook will be installed to
automatically insert a Change-Id into the commit message if one does
not already exist, so trigger the hook by performing a no-op 'git
commit --amend'.

Change-Id: Ie5e83c529b83c9f3d269df6742a59e965dbc6407
Reviewed-on: http://gerrit.openafs.org/5797
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>