dirstate: check mtime when adding to _lastnormal
- consistently use mtime as mapped to dirstate granularity (needed for
filesystems like NTFS, which have sub-second resolution)
- no need to add files with mtime < _lastnormaltime
- improve comments
http://mercurial.selenic.com/bts/issue436
$ hg init
$ hg -v log -v
$ hg -v log -v x