This will fix #919

@srajko can you add a test to confirm that the callback is indeed being throttled? It could be another clone test.

The changes are as follows:

• Progress callbacks (git_checkout_progress_cb, git_diff_file_cb, git_stash_apply_progress_cb, git_transfer_progress_cb) now have default throttling of ~1 callback per 100ms
• All callbacks provided through configuration objects can now be throttled - instead of passing a function, pass an object { callback, throttle } where throttle is the throttle period (~1 callback per throttle period, in ms). Passing throttle: 0 disables throttling.

See commit descriptions for a rundown.

👍 nice job! Thanks @srajko