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/Config.hs

@@ -50,14 +50,14 @@ optParser _ = setconfig <|> getconfig <|> unsetconfig
 seek :: Action -> CommandSeek
 seek (SetConfig name val) = commandAction $ do
 	allowMessages
-	showStart name val
+	showStart' name (Just val)
 	next $ next $ do
 		setGlobalConfig name val
 		setConfig (ConfigKey name) val
 		return True
 seek (UnsetConfig name) = commandAction $ do
 	allowMessages
-	showStart name "unset"
+	showStart' name (Just "unset")
 	next $ next $ do
 		unsetGlobalConfig name
 		unsetConfig (ConfigKey name)