show-branch: usability updates.

show-branch: usability updates.

This does three things:

 . It simplifies the logic to handle the case in which no
   refs are given on the command line, and fixes the bug
   when only "--heads" is specified.  Earlier we showed
   them twice.

 . It avoids to add the same ref twice.

 . It sorts the glob result (e.g. "git show-branch
   'tags/v1.0*'") according to a more version friendly
   sort order.

Signed-off-by: Junio C Hamano <junkio@cox.net>