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/Merge.hs

@@ -23,7 +23,7 @@ seek _ = do
 
 mergeBranch :: CommandStart
 mergeBranch = do
-	showStart "merge" "git-annex"
+	showStart' "merge" (Just "git-annex")
 	next $ do
 		Annex.Branch.update
 		-- commit explicitly, in case no remote branches were merged