pre-upload: permit s-o-b tags to follow Change-Id

The git commit -s flag will often append the signed-off-by tag.  Allow
that only to follow Change-Id to keep things simple for devs.

BUG=None
TEST=`./pre-upload_unittest.py` passes

Change-Id: I0f47dc395b8ea453789a004f08339e6a342bb215
Reviewed-on: https://chromium-review.googlesource.com/231244
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
2 files changed