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.
repoview: protect `base` computation from weird phase root

If for some reason the phase roots contains nullid, the set of filtered revs
will contains -1. That confuse Mercurial a lot. In particular this corrupt the
branchcache.

Standard code path does not result in nullid phase root. It can only result from
altered `.hg/store/phaseroots` or buggy extension. However better safe than
sorry.