This makes it easier to restart an aborted merge.

R=machenbach@chromium.org
TEST=ran tools/release/script_test.py locally

Bug: v8:7410
Change-Id: I26b1b09e0000c9c689ad80be32ced9e35634bcec
Reviewed-on: https://chromium-review.googlesource.com/905665
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51143}