Stop using git-rev-parse to parse the search ref.
Stop using git-rev-parse to parse the search ref.

This lets us run with only an index file.

paulproteus/livegrep,paulproteus/livegrep,lekkas/livegrep,paulproteus/livegrep,wfxiang08/livegrep,wfxiang08/livegrep,lekkas/livegrep,paulproteus/livegrep,wfxiang08/livegrep,lekkas/livegrep,wfxiang08/livegrep,wfxiang08/livegrep,paulproteus/livegrep,lekkas/livegrep,lekkas/livegrep,lekkas/livegrep,wfxiang08/livegrep,paulproteus/livegrep