git: add helper to determine if something's a full SHA

BUG=b:333462347
TEST=repo upload

Change-Id: I83ba2269e45c8a14ef548a8c1f4b6a352356e23e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/5458711
Commit-Queue: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
Reviewed-by: Ryan Beltran <ryanbeltran@chromium.org>
2 files changed