Collapse commit list by default in sheriff-o-matic.

This CL changes the sheriff-o-matic to collapse the commit list by default.
Each repository has a button you can press to expand the list of commits.

BUG=399715
NOTRY=true

Review URL: https://codereview.chromium.org/441393002

git-svn-id: svn://svn.chromium.org/blink/trunk@180386 bbb929c8-8fbe-4397-9dbb-9b2b20218538