fetch: avoid unpacking headers in object existence check

When updating local refs after the fetch has transferred all objects, we
do an object existence test as a safety guard to avoid updating a ref to
an object which we don't have. We do so via `oid_object_info()`: if it
returns an error, then we know the object does not exist.

One side effect of `oid_object_info()` is that it parses the object's
type, and to do so it must unpack the object header. This is completely
pointless: we don't care for the type, but only want to assert that the
object exists.

Refactor the code to use `repo_has_object_file()`, which both makes the
code's intent clearer and is also faster because it does not unpack
object headers. In a real-world repo with 2.3M refs, this results in a
small speedup when doing a mirror-fetch:

    Benchmark #1: HEAD~: git-fetch
      Time (mean ± σ):     33.686 s ±  0.176 s    [User: 30.119 s, System: 5.262 s]
      Range (min … max):   33.512 s … 33.944 s    5 runs

    Benchmark #2: HEAD: git-fetch
      Time (mean ± σ):     31.247 s ±  0.195 s    [User: 28.135 s, System: 5.066 s]
      Range (min … max):   30.948 s … 31.472 s    5 runs

    Summary
      'HEAD: git-fetch' ran
        1.08 ± 0.01 times faster than 'HEAD~: git-fetch'

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

builtin/fetch.c

@@ -846,13 +846,11 @@ static int update_local_ref(struct ref *ref,
 			    int summary_width)
 {
 	struct commit *current = NULL, *updated;
-	enum object_type type;
 	struct branch *current_branch = branch_get(NULL);
 	const char *pretty_ref = prettify_refname(ref->name);
 	int fast_forward = 0;
 
-	type = oid_object_info(the_repository, &ref->new_oid, NULL);
-	if (type < 0)
+	if (!repo_has_object_file(the_repository, &ref->new_oid))
 		die(_("object %s not found"), oid_to_hex(&ref->new_oid));
 
 	if (oideq(&ref->old_oid, &ref->new_oid)) {