mwindow: Fix a bug in the LRU window finding code

This change now updates the `lru_window` variable to match the current
file's MRU window. This makes it such that it doesn't always choose the
file that happened to come last in the list of window files, and instead
should now correctly choose the file with the least-recently-used one.

src/mwindow.c

@@ -312,9 +312,11 @@ static int git_mwindow_find_lru_file_locked(git_mwindow_file **out)
 				current_file, &mru_window, NULL, true, GIT_MWINDOW__MRU)) {
 			continue;
 		}
-		if (!lru_window || lru_window->last_used > mru_window->last_used)
+		if (!lru_window || lru_window->last_used > mru_window->last_used) {
+			lru_window = mru_window;
 			lru_file = current_file;
 		}
+	}
 
 	if (!lru_file) {
 		git_error_set(GIT_ERROR_OS, "failed to close memory window file; couldn't find LRU");