odb_pack: try lookup before refreshing packs

This reduces the rate of syscalls for the common case of sequences of
object reads from the same pack.

Best of 5 timings for libgit2_clar before this patch:
real    0m5.375s
user    0m0.392s
sys     0m3.564s

After applying this patch:
real    0m5.285s
user    0m0.356s
sys     0m3.544s

0.6% improvement in system time.
9.2% improvement in user time.
1.7% improvement in elapsed time.

Confirmed a 0.6% reduction in number of system calls with strace.

Expect greater improvement for graph-traversal with large packs.

src/odb_pack.c

@@ -268,13 +268,13 @@ static int pack_entry_find(struct git_pack_entry *e, struct pack_backend *backen
 	int error;
 	unsigned int i;
 
-	if ((error = packfile_refresh_all(backend)) < 0)
-		return error;
-
 	if (backend->last_found &&
 		git_pack_entry_find(e, backend->last_found, oid, GIT_OID_HEXSZ) == 0)
 		return 0;
 
+	if ((error = packfile_refresh_all(backend)) < 0)
+		return error;
+
 	for (i = 0; i < backend->packs.length; ++i) {
 		struct git_pack_file *p;