Use actual diff information instead of re-scanning the whole tree

[jamwt]

Right now, to keep things simple, dryad uses the GitHub API to grab the entire file tree for whatever commit is at HEAD. It then checks every single file against the database to see if anything has changed.

A more optimal approach (especially for big repositories) would be to consume each commit on the target branch and actually use the commit's information about what file(s) changed. Then only reindex (or delete) those.