packfile: replace lseek+read with pread

We already have pread emulation for portability, so there's
there's no reason to make two syscalls where one suffices.

Furthermore, readers of the packfile will be using mmap
(or pread to emulate mmap), anyways, so the file description
offset does not matter in this case.

Signed-off-by: Eric Wong <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Eric Wong

packfile.c

@@ -555,9 +555,8 @@ static int open_packed_git_1(struct packed_git *p)
 			     " while index indicates %"PRIu32" objects",
 			     p->pack_name, ntohl(hdr.hdr_entries),
 			     p->num_objects);
-	if (lseek(p->pack_fd, p->pack_size - hashsz, SEEK_SET) == -1)
-		return error("end of packfile %s is unavailable", p->pack_name);
-	read_result = read_in_full(p->pack_fd, hash, hashsz);
+	read_result = pread_in_full(p->pack_fd, hash, hashsz,
+					p->pack_size - hashsz);
 	if (read_result < 0)
 		return error_errno("error reading from %s", p->pack_name);
 	if (read_result != hashsz)