Merge branch 'jk/fewer-pack-rescan'

Internaly we use 0{40} as a placeholder object name to signal the
codepath that there is no such object (e.g. the fast-forward check
while "git fetch" stores a new remote-tracking ref says "we know
there is no 'old' thing pointed at by the ref, as we are creating
it anew" by passing 0{40} for the 'old' side), and expect that a
codepath to locate an in-core object to return NULL as a sign that
the object does not exist.  A look-up for an object that does not
exist however is quite costly with a repository with large number
of packfiles.  This access pattern has been optimized.

* jk/fewer-pack-rescan:
  sha1_file: fast-path null sha1 as a missing object
  everything_local: use "quick" object existence check
  p5551: add a script to test fetch pack-dir rescans
  t/perf/lib-pack: use fast-import checkpoint to create packs
  p5550: factor out nonsense-pack creation

sha1_file.c

@@ -1164,6 +1164,9 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi,
 				    lookup_replace_object(sha1) :
 				    sha1;
 
+	if (is_null_sha1(real))
+		return -1;
+
 	if (!oi)
 		oi = &blank_oi;