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/SetPresentKey.hs

@@ -23,7 +23,7 @@ seek = withWords start
 
 start :: [String] -> CommandStart
 start (ks:us:vs:[]) = do
-	showStart' "setpresentkey" k (mkActionItem k)
+	showStartKey "setpresentkey" k (mkActionItem k)
 	next $ perform k (toUUID us) s
   where
 	k = fromMaybe (giveup "bad key") (file2key ks)