util: Add a git commit-msg hook

util: Add a git commit-msg hook

Add a git commit-msg hook that verifies that commit messages follow
gem5 guidelines.

Commit messages must contain the following components:
    <gem5_tags>: <title>

    <description>
    <patch_tags>

<gem5_tags> are comma separated keywords (found in MAINTAINERS) that
describe which sections of gem5 are being modified by the patch.
Two special keywords can also be used to imply that the author is
looking for feedback on the way their commit was implemented (RFC),
and to inform that the commit is a work in progress (WIP).

<title> A short and concise description of the commit without trailing
whitespaces

<description> is an optional (yet highly recommended) detailed
description of the objective of the commit.

<patch_tags> describe the metadata of the commit, and most of them
are automatically added by Gerrit.

Change-Id: Ib6fb6edf6d1417bfda23729b35c5b8ed44d2cf51
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21739
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>