Add a merge-with-upstream script to toolchain-utils

Android wants to pull toolchain-utils more often. In order to make this
easier, let's have a script to do it.

BUG=None
TEST=The CL linked in the patch. Tried a few fun tricks to cover the
conditional logic, too.

Change-Id: If8de867b3711bf33309db7c64dc5b67e51e36423
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/1841586
Reviewed-by: Stephen Hines <srhines@google.com>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed