showStart variant for when there's no worktree file

Clean up some uses of showStart with "" for the file,
or in some cases, a non-filename description string. That would
generate bad json, although none of the commands doing that
supported --json.

Using "" for the file resulted in output like "foo  rest";
now the extra space is eliminated.

This commit was sponsored by Fernando Jimenez on Patreon.

Command/Unused.hs

@@ -70,7 +70,7 @@ start o = do
 		Just "." -> (".", checkUnused refspec)
 		Just "here" -> (".", checkUnused refspec)
 		Just n -> (n, checkRemoteUnused n refspec)
-	showStart "unused" name
+	showStart' "unused" (Just name)
 	next perform
 
 checkUnused :: RefSpec -> CommandPerform
@@ -338,5 +338,5 @@ startUnused message unused badunused tmpunused maps n = search
 		case M.lookup n m of
 			Nothing -> search rest
 			Just key -> do
-				showStart message (show n)
+				showStart' message (Just $ show n)
 				next $ a key