Fix v8rel script after adding roll branches.

Fix v8rel script after adding roll branches.

Use git hashes as keys when referring to releases, not
pure commit position numbers as those are not unique.

Better bailout to only check for recent releases.

Add new branching method with a roll branch in heads to
the v8rel test case.

Fix the way v8 is referred to in DEPS files in the test
cases. The test still had svn-number style.

BUG=chromium:451975
TBR=tandrii@chromium.org
NOTRY=true
LOG=n

TEST=./script_test.py

Review URL: https://codereview.chromium.org/928243002

Cr-Commit-Position: refs/heads/master@{#26669}