misc: Updated git-commit-msg.py to print rejected commit

misc: Updated git-commit-msg.py to print rejected commit

A rejected commit is now printed to stdout. This adds an additional
level of security if the ".git/COMMIT_EDITMSG" is deleted/overwritten.

Jira: https://gem5.atlassian.net/browse/GEM5-321
Change-Id: I87d463f7c40024d68bf78705f0d02fcea9f0eeeb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25343
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>