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/ResolveMerge.hs

@@ -23,7 +23,7 @@ seek = withNothing start
 
 start :: CommandStart
 start = do
-	showStart "resolvemerge" ""
+	showStart' "resolvemerge" Nothing
 	us <- fromMaybe nobranch <$> inRepo Git.Branch.current
 	d <- fromRepo Git.localGitDir
 	let merge_head = d </> "MERGE_HEAD"