[review_comments_retriever] Avoid starting from scratch on each run

[suhaibmujahid]

Currently, each time we run the script, it starts retrieving comments from scratch. Ideally, the script should start from where it left off in the previous run.

bugbug/scripts/review_comments_retriever.py

Lines 16 to 17 in 0d14e5f

	# TODO: support resuming from where last run left off. We should run it from
	# scratch only once. Following runs should add only new comments.