e.g. when specifying both '--revision src@<something>' and
'--revision <src_gerrit_repo>@<some_gerrit_ref>', gclient will sync to
'<some_gerrit_ref>' instead of '<something>'.

This is useful when specifying which gerrit change to patch, which
should take priority over other revision specifications.

Bug: chromium:643346
Change-Id: Ibc21ede355b56e4da966f38f144ce6f6f1743403
Reviewed-on: https://chromium-review.googlesource.com/949981
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>