Added re-read refs command, and display all refs.

These are features requested by Junio.  Any plain file under .git/refs
whose contents start with 40 hex characters is taken as a reference
and displayed like a head but with a light blue background (unless it
is in .git/refs/tags or .git/refs/heads, in which case it is displayed
as before).  There is now a "Reread references" menu item in the File
menu which re-reads all the plain files under .git/refs and redisplays
any references that have changed.