Suggestion to add incremental sync for github pull_request_commits stream

[emily25inGoFreight]

ref: https://airbytehq.slack.com/archives/C021JANJ6TY/p1668779910097399

Currently pull_request_commits only support full-refresh, which cause lots of API calls and takes a lot of time.
I believe the way that only fetch pull_request_commits for pull-requests updated after the last sync would be helpful.