bash: offer refs for 'git bisect start'

The completion script only offered path completion after 'git bisect
start', although bad and good refs could also be specified before the
doubledash.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -912,7 +912,7 @@ _git_bisect ()
 	fi
 
 	case "$subcommand" in
-	bad|good|reset|skip)
+	bad|good|reset|skip|start)
 		__gitcomp "$(__git_refs)"
 		;;
 	*)