vfs: create copy at renaming to avoid file stat ambiguity if needed In order to fix issue5418, bff5ccbe5ead made vfs.rename(checkambig=True) omit advancing mtime of renamed file, if renamed file is owned by another (EPERM is raised in this case). But this omission causes rewinding mtime at restoration in such situation, and makes avoiding file stat ambiguity difficult, because ExactCacheValidationPlan assumes that mtime should be advanced, if a file is changed in same ctime. https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan Ambiguity of file stat also requires issue5584 to be fixed with other than file stat, but "hash of file", "generation ID" and so on were already rejected ideas (please see original RFC linked from "Outline of issue" in ExactCacheValidationPlan page). This omission occurs: - only for non append-only files (dirstate, bookmarks, and phaseroots), and - only if previous transaction is rollbacked by another user The latter means "sharing a repository clone via group permission". This is reasonable usecase, but not ordinary for many users, IMHO. "hg rollback" itself has been deprecated since Mercurial 2.7, too. Therefore, increasing the cost at rollbacking previous transaction executed by another a little seems reasonable, for avoidance of file stat ambiguity. This patch does: - create copy of (already renamed) source file, if advancing mtime fails for EPERM - rename from copied file to destination file, and - advance mtime of renamed file, which is now owned by current user This patch also factors "self.join(src)" out to reduce redundancy.