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

diff --cc builtin/gc.c
Simple merge

diff --cc builtin/prune.c
index 10b03d3,6a58e75..8f4f052
--- 1/builtin/prune.c
--- 2/builtin/prune.c
+++ b/builtin/prune.c
@@@ -119,6 -211,16 +119,9 @@@ int cmd_prune(int argc, const char **ar
  
        argc = parse_options(argc, argv, prefix, options, prune_usage, 0);
  
 -      if (do_prune_worktrees) {
 -              if (argc)
 -                      die(_("--worktrees does not take extra arguments"));
 -              prune_worktrees();
 -              return 0;
 -      }
 -
+       if (repository_format_precious_objects)
+               die(_("cannot prune in a precious-objects repo"));
+ 
        while (argc--) {
                unsigned char sha1[20];
                const char *name = *argv++;

diff --cc builtin/repack.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc environment.c
Simple merge

diff --cc setup.c
Simple merge