bash: offer refs instead of filenames for 'git revert'

The completion script for 'git revert' currently offers options and
filenames.  However, 'git revert' doesn't take any filenames from the
command line, but a single commit.  Therefore, it's more sane to offer
refs instead.
Signed-off-by: NSZEDER Gábor <szeder@ira.uka.de>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1344,7 +1344,7 @@ _git_revert ()
 		return
 		;;
 	esac
-	COMPREPLY=()
+	__gitcomp "$(__git_refs)"
 }
 
 _git_rm ()