vfs: replace avoiding ambiguity in abstractvfs.rename with _avoidambig This centralizes common logic to forcibly avoid file stat ambiguity into _avoidambig(), which was introduced by previous patch.

vfs: copy if EPERM to avoid file stat ambiguity forcibly at closing Now, files (to be truncated) are opened with checkambig=True, only if localrepository caches it. Therefore, straightforward "copy if EPERM" is always reasonable to avoid file stat ambiguity at closing. This patch makes checkambigatclosing close wrapper copy the target file, and advance mtime on it after renaming, if EPERM. This can avoid file stat ambiguity, even if the target file is owned by another (see issue5418 and issue5584 for detail). This patch factors main logic out instead of changing checkambigatclosing._checkambig() directly, in order to reuse it.

transaction: apply checkambig=True only on limited files for similarity Now, transaction can determine whether avoidance of file stat ambiguity is needed for each files, by blacklist "checkambigfiles". For similarity to truncation in _playback(), this patch apply checkambig=True only on limited files in code paths below. - restoring files by util.copyfile(), in _playback() (checkambigfiles itself is examined at first, because it as a keyword argument might be None) - writing files at finalization of transaction, in _generatefiles() This patch reduces cost of checking stat at writing out and restoring files, which aren't filecache-ed.