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/ImportFeed.hs

@@ -72,7 +72,7 @@ seek o = do
 
 start :: ImportFeedOptions -> Cache -> URLString -> CommandStart
 start opts cache url = do
-	showStart "importfeed" url
+	showStart' "importfeed" (Just url)
 	next $ perform opts cache url
 
 perform :: ImportFeedOptions -> Cache -> URLString -> CommandPerform