merge-recursive: avoid confusing logic in was_dirty()

It took this developer more than a moment to verify that was_dirty()
really returns 0 (i.e. "false") if the file was not even tracked. In
other words, the `dirty` variable that was initialized to 1 (i.e.
"true") and then negated to be returned was not helping readability.

The same holds for the final return: rather than assigning the value to
return to `dirty` and then *immediately* returning that, we can simplify
it to a single statement.

merge-recursive.c

@@ -875,15 +875,13 @@ static int would_lose_untracked(struct merge_options *opt, const char *path)
 static int was_dirty(struct merge_options *opt, const char *path)
 {
 	struct cache_entry *ce;
-	int dirty = 1;
 
 	if (opt->priv->call_depth || !was_tracked(opt, path))
-		return !dirty;
+		return 0;
 
 	ce = index_file_exists(opt->priv->unpack_opts.src_index,
 			       path, strlen(path), ignore_case);
-	dirty = verify_uptodate(ce, &opt->priv->unpack_opts) != 0;
-	return dirty;
+	return verify_uptodate(ce, &opt->priv->unpack_opts) != 0;
 }
 
 static int make_room_for_path(struct merge_options *opt, const char *path)