FR: Support indexing of branches

[ultravigilante]

Sometimes I keep variants of my code on branches without intending to merge it into the default branch. Would be cool to see those in Sourcebot too.

[zapp42]

There is the "branch:xxxxx" search keyword, but I don't think it is working.

[brendan-kellam]

Yea there is the branch filter, but we only index HEAD atm so it's not too useful currently