Revert "move pack creation to version 3"

This reverts commit 16854571aa.
Git as recent as v1.1.6 do not understand version 3 delta.

v1.2.0 is Ok and I personally would say it is old enough, but
the improvement between version 2 and version 3 delta is not
bit enough to justify breaking older clients.

We should resurrect this later, but when we do so we shold
make it conditional.

Signed-off-by: Junio C Hamano <junkio@cox.net>

pack.h

@@ -7,7 +7,7 @@
  * Packed object header
  */
 #define PACK_SIGNATURE 0x5041434b	/* "PACK" */
-#define PACK_VERSION 3
+#define PACK_VERSION 2
 #define pack_version_ok(v) ((v) == htonl(2) || (v) == htonl(3))
 struct pack_header {
 	unsigned int hdr_signature;