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
Fix -Wunused-result warning for chdir on linux.
Merge pull request #785 from nico/unused

Fix -Wunused-result warning for chdir on linux.
Update ninja-mode for emacs to handle hyphens in rule names
Merge pull request #786 from donkopotamus/patch-1

Update ninja-mode for emacs to handle hyphens in rule names
Rename -d nowinstatcache to -d nostatcache; might become useful elsewhere (#787)
Merge pull request #789 from nico/dontwin

Rename -d nowinstatcache to -d nostatcache; might become useful elsewhere (#787)