pack: move open_pack_index(), parse_pack_index()

alloc_packed_git() in packfile.c is duplicated from sha1_file.c. In a
subsequent commit, alloc_packed_git() will be removed from sha1_file.c.

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

builtin/count-objects.c

@@ -10,6 +10,7 @@
 #include "builtin.h"
 #include "parse-options.h"
 #include "quote.h"
+#include "packfile.h"
 
 static unsigned long garbage;
 static off_t size_garbage;