util: Filter out the For-Upstream commit message tag

util: Filter out the For-Upstream commit message tag

Some internal review flows, e.g. the flow used by ARM Research, uses
the For-Upstream tag to indicate that a change has been approved for
upstream. This tag isn't meaningful outside of the internal review
system. Remove it automatically when running the maintainer script to
prepare patches to be posted upstream.

Change-Id: Ie3745d0e8ad7a1bfddc5ec68ff3e6ff3f91ca8d8
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5141
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>