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

Documentation/technical/api-ref-iteration.txt

@@ -32,11 +32,8 @@ Iteration functions
 
 * `for_each_glob_ref_in()` the previous and `for_each_ref_in()` combined.
 
-* `head_ref_submodule()`, `for_each_ref_submodule()`,
-  `for_each_ref_in_submodule()`, `for_each_tag_ref_submodule()`,
-  `for_each_branch_ref_submodule()`, `for_each_remote_ref_submodule()`
-  do the same as the functions described above but for a specified
-  submodule.
+* Use `refs_` API for accessing submodules. The submodule ref store could
+  be obtained with `get_submodule_ref_store()`.
 
 * `for_each_rawref()` can be used to learn about broken ref and symref.