vfs: make rename avoid ambiguity of file stat if needed In some cases below, renaming from backup is used to restore original contents of a file. If renaming keeps ctime, mtime and size of a file, restoring is overlooked, and old contents cached before restoring isn't invalidated as expected. - failure of transaction before closing (only from '.hg/journal.dirstate') - rollback of previous transaction (from '.hg/undo.*') - failure in dirstateguard scope (from '.hg/dirstate.SUFFIX') To avoid such problem, this patch makes vfs.rename() avoid ambiguity of file stat, if needed. Ambiguity check is executed, only if: - checkambig=True is specified (not all renaming needs ambiguity check), and - destination file exists before renaming This patch is a part of preparation for "Exact Cache Validation Plan": https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan