Added rsync monitoring overhead benchmark.
Added rsync monitoring overhead benchmark.

This is a relatively crude benchmark, but it at least gives us a
ballpark estimate of the rsync monitoring overhead. The bare minimum
callback gives an overhead of around 15 ns per call. For 100,000
files, that works out to about 1.5 ms. This is also probably not our
culprit.

havoc-io/mutagen,havoc-io/mutagen