commit-graph: refactor parse_commit_graph() to take a repository

Refactor `parse_commit_graph()` so that it takes a repository instead of
taking repository settings. On the one hand this allows us to get rid of
instances where we access `the_hash_algo` by using the repository's hash
algorithm instead. On the other hand it also allows us to move the call
of `prepare_repo_settings()` into the function itself.

Note that there's one small catch, as the commit-graph fuzzer calls this
function directly without having a fully functional repository at hand.
And while the fuzzer already initializes `the_repository` with relevant
info, the call to `prepare_repo_settings()` would fail because we don't
have a fully-initialized repository.

Work around the issue by also settings `settings.initialized` to pretend
that we've already read the settings.

While at it, remove the redundant `parse_commit_graph()` declaration in
the fuzzer. It was added together with aa658574bf (commit-graph, fuzz:
add fuzzer for commit-graph, 2019-01-15), but as we also declared the
same function in "commit-graph.h" it wasn't ever needed.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.h

@@ -128,7 +128,7 @@ struct repo_settings;
  * Callers should initialize the repo_settings with prepare_repo_settings()
  * prior to calling parse_commit_graph().
  */
-struct commit_graph *parse_commit_graph(struct repo_settings *s,
+struct commit_graph *parse_commit_graph(struct repository *r,
 					void *graph_map, size_t graph_size);
 
 /*