packfile: skip unpacking object header for disk size requests

While most of the object info requests for a packed object require us to
unpack its headers, reading its disk size doesn't. We still unpack the
object header in that case though, which is unnecessary work.

Skip reading the header if only the disk size is requested. This leads
to a small speedup when reading disk size, only. The following benchmark
was done in the Git repository:

    Benchmark 1: ./git rev-list --disk-usage HEAD (rev = HEAD~)
      Time (mean ± σ):     105.2 ms ±   0.6 ms    [User: 91.4 ms, System: 13.3 ms]
      Range (min … max):   103.7 ms … 106.0 ms    27 runs

    Benchmark 2: ./git rev-list --disk-usage HEAD (rev = HEAD)
      Time (mean ± σ):      96.7 ms ±   0.4 ms    [User: 86.2 ms, System: 10.0 ms]
      Range (min … max):    96.2 ms …  98.1 ms    30 runs

    Summary
      ./git rev-list --disk-usage HEAD (rev = HEAD) ran
        1.09 ± 0.01 times faster than ./git rev-list --disk-usage HEAD (rev = HEAD~)

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

Author: pks-t

packfile.c

@@ -1586,7 +1586,7 @@ int packed_object_info(struct repository *r, struct packed_git *p,
 	struct pack_window *w_curs = NULL;
 	unsigned long size;
 	off_t curpos = obj_offset;
-	enum object_type type;
+	enum object_type type = OBJ_NONE;
 	int ret;
 
 	/*
@@ -1598,7 +1598,7 @@ int packed_object_info(struct repository *r, struct packed_git *p,
 						      &type);
 		if (!*oi->contentp)
 			type = OBJ_BAD;
-	} else {
+	} else if (oi->sizep || oi->typep || oi->delta_base_oid) {
 		type = unpack_object_header(p, &w_curs, &curpos, &size);
 	}
 
@@ -1662,6 +1662,9 @@ int packed_object_info(struct repository *r, struct packed_git *p,
 	oi->u.packed.pack = p;
 
 	switch (type) {
+	case OBJ_NONE:
+		oi->u.packed.type = PACKED_OBJECT_TYPE_UNKNOWN;
+		break;
 	case OBJ_REF_DELTA:
 		oi->u.packed.type = PACKED_OBJECT_TYPE_REF_DELTA;
 		break;