Proposed/fix restat rebuild if deps missing

Fixing https://github.com/martine/ninja/issues/603, + two micro fixes in restat code on the way.

Apparently, the problem origins to my fix to "consider target dirty if depfile is missing" (bcc8ad1369ae3d90631729d29cc83c377f44535e), which was not working correctly with restat rules cleaning nodes. Switching to deps only triggered an easily observable issue.