Rewrite the Pack backend

The new pack backend is an adaptation of the original git.git code in
`sha1_file.c`. It's slightly faster than the previous version and
severely less memory-hungry.

The call-stack of a normal pack backend query has been properly
documented in the top of the header for future reference. And by
properly I mean with ASCII diagrams 'n shit.

src/util.h

@@ -2,6 +2,8 @@
 #define INCLUDE_util_h__
 
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
+#define bitsizeof(x)  (CHAR_BIT * sizeof(x))
+#define MSB(x, bits) ((x) & (~0ULL << (bitsizeof(x) - (bits))))
 
 /* 
  * Don't wrap malloc/calloc.