oidmap: hash on the id, not the object

We explicitly want to hash on the id data, not the beginning of the
object data, which may contain other information in the future.

src/libgit2/oidmap.c

@@ -19,7 +19,7 @@ __KHASH_TYPE(oid, const git_oid *, void *)
 GIT_INLINE(khint_t) git_oidmap_hash(const git_oid *oid)
 {
 	khint_t h;
-	memcpy(&h, oid, sizeof(khint_t));
+	memcpy(&h, oid->id, sizeof(khint_t));
 	return h;
 }