Autocomplete branches in search. Implements feature requested in issue #223.

- The regex in the get "/autocomplete/branches" on the remote.name is not ideal. I tried using Grit to get branch names rather than remotes, but this doesn't include remote branches and searches are allowed on branches that aren't local. Maybe there's a cleaner solution to this I'm overlooking.
- Because I'm querying Grit for remotes and not branches, it includes "HEAD", which I have to exclude explicitly. Again, maybe there's a cleaner solution.
Merge pull request #303 from lyahdav/autocomplete_branches

Autocomplete branches in search. Implements feature requested in issue #223.