Unless single_worktree is set, --all now adds HEAD from all worktrees.
Since reachable.c code does not use setup_revisions(), we need to call
other_head_refs_submodule() explicitly there to have the same effect on
"git prune", so that we won't accidentally delete objects needed by some
other HEADs.
A new FIXME is added because we would need something like
int refs_other_head_refs(struct ref_store *, each_ref_fn, cb_data);
in addition to other_head_refs() to handle it, which might require
int get_submodule_worktrees(const char *submodule, int flags);
It could be a separate topic to reduce the scope of this one.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>