Add --refs to bot_update
Add --refs to bot_update

This generalizes --with_branch_heads into --refs.

This is here because some gerrit projects needs to check out try refs, which means their recipes would pass --refs +refs/changes/11/11111/1 into bot_update

BUG=

Review URL: https://codereview.chromium.org/606143002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@292194 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build