scons: Automatically add a git commit message hook

Gerrit requires that all commit messages have a Change-Id tag. This
tag is added automatically by a commit message hook in Git. Include
the default Gerrit commit message hook and add it automatically using
scons to make life easier for everyone.

Change-Id: I1270fbaaadf6ed151bddf14521a38e0c1a02d131
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2166
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>