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/Map.hs

@@ -189,7 +189,7 @@ same a b
 {- reads the config of a remote, with progress display -}
 scan :: Git.Repo -> Annex Git.Repo
 scan r = do
-	showStart "map" $ Git.repoDescribe r
+	showStart' "map" (Just $ Git.repoDescribe r)
 	v <- tryScan r
 	case v of
 		Just r' -> do