LU-15511 misc: allow Lustre-change and Lustre-commit

Allow the Lustre-change:, Lustre-commit:, Linux-commit: labels in
the signoff section.  Verify that Lustre-change: has a valid-looking
Gerrit URL in "permalink" format "https://review.whamcloud.com/nnnnn".
Verify -commit: labels have a valid-looking Git hashes (40 hex chars),
though the actual hash cannot be verified because it may be from a
different Git repository.

Shorten the help message in case of error, and add "commit-msg --help"
to print the full commit format help.

Lustre-change: https://review.whamcloud.com/46420
Lustre-commit: TBD (4de05884f20089c23556f860381f17203578220e)
Linux-commit: cada0382e5087aeb0bf516cdec1aec9a0c158151
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I866ab91b33f6e52ff893344df74af243903ebbe5