Parse diffs on demand for better performance #254

Currently we're unnecessarily parsing git diff output for all changed files when there is a file system change. That is, if we have 10 changed files and only one is modified, we get the diff output for all files, parse it, and create FilePatch objects for each of them when really we only need to do those things for the single file that changed.

To implement this we would check the nsfw change event to determine which file changed (if it's the index, we need to use the blob sha to determine which file changed) and only update the FilePatch corresponding to that file. Since on linux we can't watch entire directories (see #253), we may still want to periodically update all of the FilePatches.

Note: testing for this must be done on Mac, Windows, and Linux