When set_output_commit is True and the repo associated with the
got_revision property does not use commit positions, buiders whose
gclient config uses RevisionFallbackChain for that repo will raise an
exception when attempting set the output commit because a string is
expected for the revision value, not a RevisionFallbackChain object.
This changes the set_output_commit code to resolve the revision so that
the output commit can be set in those cases.

Bug: 1159749, 1141886
Change-Id: Iddf1d49871028d9500eefb0eed07e36ce7e6b0d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2612223
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>