Remove dependency on the_repository and other globals from the
commit-graph code, and other changes unrelated to de-globaling.
* ps/commit-graph-wo-globals:
commit-graph: stop passing in redundant repository
commit-graph: stop using `the_repository`
commit-graph: stop using `the_hash_algo`
commit-graph: refactor `parse_commit_graph()` to take a repository
commit-graph: store the hash algorithm instead of its length
commit-graph: stop using `the_hash_algo` via macros