Summary of changes

• Removed access to the diff_so_far param in git_diff_notify_cb and git_diff_progress_cb
• Changed FilterSource.prototype.repo to async to prevent segfaults on filters that run during Submodule.status
• Clean up deprecation messages to inform users of what was deprecated, not just what users should switch to
• When installing on a machine that has yarn and does not have npm, the preinstall script should succeed now
• ceiling_dirs is now an optional parameter to Repository.discover

Merged PRs into NodeGit

• Clean up some dangerous memory accesses in callbacks #1642
• Output the item that was deprecated when giving deprecation notice #1643
• Don't fail yarn installs when we can't find npm #1644
• ceiling_dirs parameter in Repository.discover is optional #1245