make Stat() a const method
do not delete files from the logs that still exist on disk

This is to keep the possibility of maybe having a tool that deletes
old files in the future, or for having a tool which exposes this
information to generators so they can do that.

See https://github.com/martine/ninja/pull/697#issuecomment-37140762
and the discussion on #762.

Idea from @maximuska!
Merge pull request #784 from nico/statkeep

do not delete files from the logs that still exist on disk