got_revision and got_revision_cp point to the commit that
Build.output.gitiles_commit should point to.
Therefore, derive output commit from these properties.

Derive output ref from got_revision_cp if it available.
Otherwise, try to read it from input commit

Bug: 940214
Change-Id: I108d782ad93c70114581830d7427b95d3efd9d98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1588506Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>