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>

util: Add maintainer tools to create upstream patches

This changeset adds a maintainer script, create_patches.sh, that can
be used to prepare for upstream from a git repository. The script can
be used to generate patches in Mercurial or git format. The commit
messages in the exported patches are all filtered, see
upstream_msg_filter.sed, to ensure that irrelevant meta data isn't
included in the upstream commit.

Kudos to Curtis Dunham and Nikos Nikoleris for reviews and usability
enhancements for earlier versions of this patch.

Change-Id: Ia4cd089a32834b5e046ef58c0a173ca285b77bca
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>