set_work_tree: use chdir_notify

set_work_tree: use chdir_notify

When we change to the top of the working tree, we manually
re-adjust $GIT_DIR and call set_git_dir() again, in order to
update any relative git-dir we'd compute earlier.

Instead of the work-tree code having to know to call the
git-dir code, let's use the new chdir_notify interface.
There are two spots that need updating, with a few
subtleties in each:

  1. the set_git_dir() code needs to chdir_notify_register()
     so it can be told when to update its path.

     Technically we could push this down into repo_set_gitdir(),
     so that even repository structs besides the_repository
     could benefit from this. But that opens up a lot of
     complications:

      - we'd still need to touch set_git_dir(), because it
        does some other setup (like setting $GIT_DIR in the
        environment)

      - submodules using other repository structs get
        cleaned up, which means we'd need to remove them
        from the chdir_notify list

      - it's unlikely to fix any bugs, since we shouldn't
        generally chdir() in the middle of working on a
        submodule

  2. setup_work_tree now needs to call chdir_notify(), and
     can lose its manual set_git_dir() call.

     Note that at first glance it looks like this undoes the
     absolute-to-relative optimization added by 044bbbcb63
     (Make git_dir a path relative to work_tree in
     setup_work_tree(), 2008-06-19). But for the most part
     that optimization was just _undoing_ the
     relative-to-absolute conversion which the function was
     doing earlier (and which is now gone).

     It is true that if you already have an absolute git_dir
     that the setup_work_tree() function will no longer make
     it relative as a side effect. But:

       - we generally do have relative git-dir's due to the
         way the discovery code works

       - if we really care about making git-dir's relative
         when possible, then we should be relativizing them
         earlier (e.g., when we see an absolute $GIT_DIR we
         could turn it relative, whether we are going to
         chdir into a worktree or not). That would cover all
         cases, including ones that 044bbbcb63 did not.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>