completion: unite --reuse-message and --reedit-message handling

Signed-off-by: Teemu Matilainen <teemu.matilainen@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1287,12 +1287,8 @@ _git_commit ()
 			" "" "${cur##--cleanup=}"
 		return
 		;;
-	--reuse-message=*)
-		__gitcomp "$(__git_refs)" "" "${cur##--reuse-message=}"
-		return
-		;;
-	--reedit-message=*)
-		__gitcomp "$(__git_refs)" "" "${cur##--reedit-message=}"
+	--reuse-message=*|--reedit-message=*)
+		__gitcomp "$(__git_refs)" "" "${cur#*=}"
 		return
 		;;
 	--untracked-files=*)