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/NumCopies.hs

@@ -48,7 +48,7 @@ startGet = next $ next $ do
 startSet :: Int -> CommandStart
 startSet n = do
 	allowMessages
-	showStart "numcopies" (show n)
+	showStart' "numcopies" (Just $ show n)
 	next $ next $ do
 		setGlobalNumCopies $ NumCopies n
 		return True