Revert #223, fixes #874.

No test since there's still no good way to test code in ninja.cc.  Going
forward, either move NinjaMain to its own file with tests, or create a
system that makes it possible to run integration tests on the ninja
binary.  Instead, add a comment that explains why the restat
optimization isn't done.
Merge pull request #879 from nico/rev

Revert #223, fixes #874.