Merge branch 'jk/repository-extension' into maint

Prepare for Git on-disk repository representation to undergo
backward incompatible changes by introducing a new repository
format version "1", with an extension mechanism.

* jk/repository-extension:
  introduce "preciousObjects" repository extension
  introduce "extensions" form of core.repositoryformatversion

builtin/repack.c

@@ -193,6 +193,9 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, builtin_repack_options,
 				git_repack_usage, 0);
 
+	if (delete_redundant && repository_format_precious_objects)
+		die(_("cannot delete packs in a precious-objects repo"));
+
 	if (pack_kept_objects < 0)
 		pack_kept_objects = write_bitmaps;