Fix diffing on non-GitHub and improve config
1. Remove obsolete comments.
2. Remove unused function get_pr_ref
3. Correct comment on git_sha
4. Rename commit_sha to git_sha
5. Make base_commit, pr_ref, and base_ref non-github specific and move to
CiEnvironment.
6. Hoist get_diff_base to base class to allow diffing
7. Fix downloading coverage on non-Github.
8. Add TODO so that we don't assume github actions is run on github.com
(enterprise users)
9. Rename repo_url to git_url.
10. Add missing git_url method base class.
11. Clarify what git_url, git_sha and pr_ref are for and leave TODOs about how
we can eliminate them.
12. Fix typos.
This is an entrypoint for CIFuzz users that can't build two docker images
(e.g. prow users).
Co-authored-by: jonathanmetzman <31354670+jonathanmetzman@users.noreply.github.com>
jonathanmetzman