pack: move unpack_object_header_buffer()

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

packfile.c

@@ -884,3 +884,28 @@ void reprepare_packed_git(void)
 	prepare_packed_git_run_once = 0;
 	prepare_packed_git();
 }
+
+unsigned long unpack_object_header_buffer(const unsigned char *buf,
+		unsigned long len, enum object_type *type, unsigned long *sizep)
+{
+	unsigned shift;
+	unsigned long size, c;
+	unsigned long used = 0;
+
+	c = buf[used++];
+	*type = (c >> 4) & 7;
+	size = c & 15;
+	shift = 4;
+	while (c & 0x80) {
+		if (len <= used || bitsizeof(long) <= shift) {
+			error("bad object header");
+			size = used = 0;
+			break;
+		}
+		c = buf[used++];
+		size += (c & 0x7f) << shift;
+		shift += 7;
+	}
+	*sizep = size;
+	return used;
+}