Make git pack files use little-endian size encoding

This makes it match the new delta encoding, and admittedly makes the
code easier to follow.

This also updates the PACK file version to 2, since this (and the delta
encoding change in the previous commit) are incompatible with the old
format.

pack.h

@@ -1,19 +1,26 @@
 #ifndef PACK_H
 #define PACK_H
 
+/*
+ * The packed object type is stored in 3 bits.
+ * The type value 0 is a reserved prefix if ever there is more than 7
+ * object types, or any future format extensions.
+ */
 enum object_type {
-	OBJ_NONE,
-	OBJ_COMMIT,
-	OBJ_TREE,
-	OBJ_BLOB,
-	OBJ_TAG,
-	OBJ_DELTA,
+	OBJ_EXT = 0,
+	OBJ_COMMIT = 1,
+	OBJ_TREE = 2,
+	OBJ_BLOB = 3,
+	OBJ_TAG = 4,
+	/* 5/6 for future expansion */
+	OBJ_DELTA = 7,
 };
 
 /*
  * Packed object header
  */
 #define PACK_SIGNATURE 0x5041434b	/* "PACK" */
+#define PACK_VERSION 2
 struct pack_header {
 	unsigned int hdr_signature;
 	unsigned int hdr_version;