largefiles: drop os.path.join() in subrepo archive override...
largefiles: drop os.path.join() in subrepo archive override
This is related to 41cd8171e58f, but not required for that to function correctly
because 'prefix' always ends with '/', so os.path.join(prefix, _path) simply
concatenates the strings. Take the opportunity to drop the os.path usage, and
avoid confusion by using the same pattern as the overridden subrepo method.