agnoster: speed up git prompt

Problem

On detached-refs, the git prompt is very slow because it uses a command
to output all refs, which takes 32s on the twitter internal git repo.

Solution

Use git-rev-parse instead of git-show-ref to only output the head ref.
Runtime drops to 0.03 seconds.

Note

Interestingly, git-rev-parse isn't slow without the --head arguement so
I'll probably be cooking up a patch for the git team to speed this up
eventually, but it's still pointless to sigpipe git as it walks the
filesystem.