Change pack file format. Hopefully for the last time.

This also adds a header with a signature, version info, and the number
of objects to the pack file.  It also encodes the file length and type
more efficiently.

pack.h

@@ -0,0 +1,23 @@
+#ifndef PACK_H
+#define PACK_H
+
+enum object_type {
+	OBJ_NONE,
+	OBJ_COMMIT,
+	OBJ_TREE,
+	OBJ_BLOB,
+	OBJ_TAG,
+	OBJ_DELTA,
+};
+
+/*
+ * Packed object header
+ */
+#define PACK_SIGNATURE 0x5041434b	/* "PACK" */
+struct pack_header {
+	unsigned int hdr_signature;
+	unsigned int hdr_version;
+	unsigned int hdr_entries;
+};
+
+#endif