revision: avoid hitting packfiles when commits are in commit-graph

When queueing references in git-rev-list(1), we try to optimize parsing
of commits via the commit-graph. To do so, we first look up the object's
type, and if it is a commit we call `repo_parse_commit()` instead of
`parse_object()`. This is quite inefficient though given that we're
always uncompressing the object header in order to determine the type.
Instead, we can opportunistically search the commit-graph for the object
ID: in case it's found, we know it's a commit and can directly fill in
the commit object without having to uncompress the object header.

Expose a new function `lookup_commit_in_graph()`, which tries to find a
commit in the commit-graph by ID, and convert `get_reference()` to use
this function. This provides a big performance win in cases where we
load references in a repository with lots of references pointing to
commits. The following has been executed in a real-world repository with
about 2.2 million refs:

    Benchmark #1: HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      4.458 s ±  0.044 s    [User: 4.115 s, System: 0.342 s]
      Range (min … max):    4.409 s …  4.534 s    10 runs

    Benchmark #2: HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      3.089 s ±  0.015 s    [User: 2.768 s, System: 0.321 s]
      Range (min … max):    3.061 s …  3.105 s    10 runs

    Summary
      'HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' ran
        1.44 ± 0.02 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev'

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

commit-graph.c

@@ -891,6 +891,30 @@ static int find_commit_pos_in_graph(struct commit *item, struct commit_graph *g,
 	}
 }
 
+struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id)
+{
+	struct commit *commit;
+	uint32_t pos;
+
+	if (!repo->objects->commit_graph)
+		return NULL;
+	if (!search_commit_pos_in_graph(id, repo->objects->commit_graph, &pos))
+		return NULL;
+	if (!repo_has_object_file(repo, id))
+		return NULL;
+
+	commit = lookup_commit(repo, id);
+	if (!commit)
+		return NULL;
+	if (commit->object.parsed)
+		return commit;
+
+	if (!fill_commit_in_graph(repo, commit, repo->objects->commit_graph, pos))
+		return NULL;
+
+	return commit;
+}
+
 static int parse_commit_in_graph_one(struct repository *r,
 				     struct commit_graph *g,
 				     struct commit *item)