posix: don't compare atime when determining if a file has changed A file's atime might change even if the file itself doesn't change. This might cause us to invalidate caches more often than necessary. Before this change, hg add often resulted in the dirstate being parsed twice on systems that track atime. After this change, it is only parsed once. For a repository with over 180,000 files, this speeds up hg add from 1.2 seconds to 1.0.