Temp files confound find_files() depfile

[jimporter]

If something you do (e.g. edit via emacs) creates a temp file in a folder that's been searched for files, that folder will be marked as changed, triggering regeneration of the backend build script. It would be nice if we could be smarter about this, but that'll probably require https://github.com/martine/ninja/issues/987 to be fixed first.