Fix verifying and fetching files from Bitbucket.
Review Request #10583 — Created June 11, 2019 and submitted
Bitbucket turned off their 1.0 API today, which we still used solely for
the fetching of file contents. This was a surprise, but fortunately this
API has since been added to 2.0 (previously, it was 1.0-only).
This change switches us to the 2.0 API for file fetching, and also uses
the faster metadata fetching API for verifying file existence.
Unit tests pass.
Manually tested the new URLs against real repositories, verifying that the
file fetches and validation was working correctly.