LU-11810 misc: allow Fixes: tag in commit signoff block

LU-11810 misc: allow Fixes: tag in commit signoff block

Allow the "Fixes:" tag in the signoff block of the commit message.
The Fixes: tag should contain a valid git commit hash, and be
followed by a description of the original patch.

Lustre-change: https://review.whamcloud.com/33888
Lustre-commit: 4d6bff6be51ff6e336f73b2822ac5100254b9431

Fixes: 5760b34c48b4 ("LU-1145 test: add Test-Parameters tag")
Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I49f712dd8da173510e5941b66eb050e53a1cab07
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-on: https://review.whamcloud.com/37724
Reviewed-by: Ben Evans <beevans@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>