{_resolve_fixed_revisions} resolves every revision property. After a test
failure, we don't want to resolve every revision always.

This CL allows updating a subset of revisions for a new checkout.

R=machenbach@chromium.org
TBR=tandrii@chromium.org

Bug: 950731, 982711
Change-Id: I3cdc5cdb882009886b82557427fdf9b02e860b39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1720929
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>