worktree: reject NULL worktree in get_worktree_git_dir()

worktree: reject NULL worktree in get_worktree_git_dir()

This removes the final dependence on "the_repository" in
get_worktree_git_dir(). The last commit removed only caller that
passed a NULL worktree.

get_worktree_git_dir() has the following callers:

 - branch.c:prepare_checked_out_branches() which loops over all
   worktrees.

 - builtin/fsck.c:cmd_fsck() which loops over all worktrees.

 - builtin/receive-pack.c:update_worktree() which is called from
   update() only when "worktree" is non-NULL.

 - builtin/worktree.c:validate_no_submodules() which is called from
   check_clean_worktree() and move_worktree(), both of which supply
   a non-NULL worktree.

 - reachable.c:add_rebase_files() which loops over all worktrees.

 - revision.c:add_index_objects_to_pending() which loops over all
   worktrees.

 - worktree.c:is_current_worktree() which expects a non-NULL worktree.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>