Today, whenever LUCI scheduler sets gitiles_commit, a revision property
is also always set, thus this CL will have no effect on today's production.

The goal of CL is to pave the way to avoid this duplication. For starters,
one could start real writing tests with

    + api.buildbucket.ci_build(..., revision='sha1')

without the need to duplicate the same revision as a property. All
recipes which WAI with such input are thus ready to work w/o 'revision'
property. After that, we'll be able to stop the duplication in LUCI scheduler.

R=hinoka

Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build
Change-Id: I1773b1c3d2a98a9dbd1cfdd9ac2b4fb99745ee99
Reviewed-on: https://chromium-review.googlesource.com/1123713
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>