commit-graph.c: peel refs in 'add_ref_to_set'

While iterating references (to discover the set of commits to write to
the commit-graph with 'git commit-graph write --reachable'),
'add_ref_to_set' can save 'fill_oids_from_commits()' some time by
peeling the references beforehand.

Move peeling out of 'fill_oids_from_commits()' and into
'add_ref_to_set()' to use 'peel_ref()' instead of 'deref_tag()'. Doing
so allows the commit-graph machinery to use the peeled value from
'$GIT_DIR/packed-refs' instead of having to load and parse tags.

While we're at it, discard non-commit objects reachable from ref tips.
This would be done automatically by 'fill_oids_from_commits()', but such
functionality will be removed in a subsequent patch after the call to
'lookup_commit_reference_gently' is dropped (at which point a non-commit
object in the commits oidset will become an error).

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -1327,9 +1327,13 @@ static int add_ref_to_set(const char *refname,
 			  const struct object_id *oid,
 			  int flags, void *cb_data)
 {
+	struct object_id peeled;
 	struct refs_cb_data *data = (struct refs_cb_data *)cb_data;
 
-	oidset_insert(data->commits, oid);
+	if (!peel_ref(refname, &peeled))
+		oid = &peeled;
+	if (oid_object_info(the_repository, oid, NULL) == OBJ_COMMIT)
+		oidset_insert(data->commits, oid);
 
 	display_progress(data->progress, oidset_size(data->commits));