extdiff: allow modifications in subrepos to be copied back This check was a legacy bit from when the file data was being fetched manually with 'ctx[wfn]', but archive() does that now. 49966b5ab16f seems to indicate that this avoided a problem where a merge adds a file to another branch, and that test still passes. Unfortunately, I don't see a way to create a test that modifies the file in the temporary directory before the command exits. I wonder if the os.lstat() call needs to be wrapped in an exception handler for the case where archive didn't create a file because the file didn't exist in that revision. But I wasn't able to trigger a problem without it on a real repository.