This is needed as normal chromium checkouts don't fetch
branch-heads.

These branches will also be replicated by GitHub and are
fetched when cloning from the repository.

The expected object size will be the same or smaller than
before as we currently push the same information onto the
candidates branch (which already is in refs/heads). The only
difference is the increasing number of refs in refs/heads.

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

TEST=./script_test.py
TEST=tools/release/create_release.py -r machenbach@chromium.org -a machenbach@chromium.org --dry-run

Review URL: https://codereview.chromium.org/914973002

Cr-Commit-Position: refs/heads/master@{#26629}