transaction: avoid ambiguity of file stat at restoring from backup In some cases below, copying from backup is used to restore original contents of a file, which is backuped via addfilegenerator(). If copying 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 (from '.hg/journal.backup.*') - rollback of previous transaction (from '.hg/undo.backup.*') To avoid ambiguity of file stat at restoring, this patch invokes util.copyfile() with checkambig=True. This patch is a part of "Exact Cache Validation Plan": https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan