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/Describe.hs

@@ -22,7 +22,7 @@ seek = withWords start
 
 start :: [String] -> CommandStart
 start (name:description) = do
-	showStart "describe" name
+	showStart' "describe" (Just name)
 	u <- Remote.nameToUUID name
 	next $ perform u $ unwords description
 start _ = giveup "Specify a repository and a description."