Merge branch 'nd/prune-in-worktree'

"git gc" and friends when multiple worktrees are used off of a
single repository did not consider the index and per-worktree refs
of other worktrees as the root for reachability traversal, making
objects that are in use only in other worktrees to be subject to
garbage collection.

* nd/prune-in-worktree:
  refs.c: reindent get_submodule_ref_store()
  refs.c: remove fallback-to-main-store code get_submodule_ref_store()
  rev-list: expose and document --single-worktree
  revision.c: --reflog add HEAD reflog from all worktrees
  files-backend: make reflog iterator go through per-worktree reflog
  revision.c: --all adds HEAD from all worktrees
  refs: remove dead for_each_*_submodule()
  refs.c: move for_each_remote_ref_submodule() to submodule.c
  revision.c: use refs_for_each*() instead of for_each_*_submodule()
  refs: add refs_head_ref()
  refs: move submodule slash stripping code to get_submodule_ref_store
  refs.c: refactor get_submodule_ref_store(), share common free block
  revision.c: --indexed-objects add objects from all worktrees
  revision.c: refactor add_index_objects_to_pending()
  refs.c: use is_dir_sep() in resolve_gitlink_ref()
  revision.h: new flag in struct rev_info wrt. worktree-related refs

reachable.c

@@ -10,6 +10,7 @@
 #include "progress.h"
 #include "list-objects.h"
 #include "packfile.h"
+#include "worktree.h"
 
 struct connectivity_progress {
 	struct progress *progress;
@@ -177,6 +178,7 @@ void mark_reachable_objects(struct rev_info *revs, int mark_reflog,
 
 	/* detached HEAD is not included in the list above */
 	head_ref(add_one_ref, revs);
+	other_head_refs(add_one_ref, revs);
 
 	/* Add all reflog info */
 	if (mark_reflog)