copybot: Add support for subtrees

Add support for copying:
Upstream subtree to downstream subtree
Upstream to downstream subtree
upstream subtree to downstream

BUG=b:322806402,b:273207630
TEST=Run the following command with the following configurations:
./copybot.py {upstream_url}:{upstream_branch}:{upstream_subtree} {downstream_url}:{downstream_branch}:{downstream_subtree} --dry-run
coreboot main to chromium main
coreboot main:src/mainboard/google/brox/variants/baseboard/brox/ to chromium main
coreboot main:src/mainboard/google/brox/variants/baseboard/brox/gpio.c to chromium main
coreboot main:src/mainboard/google/brox/variants/baseboard/brox/ to chromium main:src/mainboard/google/brox/variants/baseboard/brox/

Change-Id: Ic88ecb91f519edbbf6d056aaf42bc1f5083f2921
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/5297988
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Jonathon Murphy <jpmurphy@google.com>
Commit-Queue: Jonathon Murphy <jpmurphy@google.com>
1 file changed