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/VFilter.hs

@@ -21,7 +21,7 @@ seek = withWords start
 
 start :: [String] -> CommandStart
 start params = do
-	showStart "vfilter" ""
+	showStart' "vfilter" Nothing
 	withCurrentView $ \view -> do
 		let view' = filterView view $
 			map parseViewParam $ reverse params