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
$ hg init a
$ echo a > a/a
$ hg -R a ci -A -m a
adding a
$ hg clone a b
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo b > b/b
$ hg -R b ci -A -m b
adding b
$ chmod 100 a/.hg/store
$ hg -R b push a
pushing to a
abort: could not lock repository a: Permission denied
[255]
$ chmod 700 a/.hg/store