Add a $toplevel variable accessible to `git submodule foreach`, it
contains the absolute path of the top level directory (where
.gitmodules is).

This makes it possible to e.g. read data in .gitmodules from within
foreach commands. I'm using this to configure the branch names I want
to track for each submodule:

    git submodule foreach 'git checkout $(git config --file $toplevel/.gitmodules submodule.$name.branch) && git pull'

For a little history: This patch is borne out of my continuing fight
of trying to have Git track the branches of submodules, not just their
commits.

Obviously that's not how they work (they only track commits), but I'm
just interested in being able to do:

    git submodule foreach 'git pull'

Of course that won't work because the submodule is in a disconnected
head, so I first have to connect it, but connect it *to what*.

For a while I was happy with this because as fate had it, it just so
happened to do what I meant:

    git submodule foreach 'git checkout $(git describe --all --always) && git pull'

But then that broke down, if there's a tag and a branch the tag will
win out, and I can't git pull a branch:

    $ git branch -a
    * master
      remotes/origin/HEAD -> origin/master
      remotes/origin/master
    $ git tag -l
    release-0.0.6
    $ git describe --always --all
    release-0.0.6

So I figured that I might as well start tracking the branches I want
in .gitmodules itself:

    [submodule "yaml-mode"]
        path = yaml-mode
        url = git://github.com/yoshiki/yaml-mode.git
        branch = master

So now I can just do (as stated above):

    git submodule foreach 'git checkout $(git config --file $toplevel/.gitmodules submodule.$name.branch) && git pull'

Maybe there's a less painful way to do *that* (I'd love to hear about
it). But regardless of that I think it's a good idea to be able to
know what the top-level is from git submodule foreach.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>