Somebody tried "git pull" from a random place completely outside the work
tree, while exporting GIT_DIR and GIT_WORK_TREE that are set to correct
places, e.g.

    GIT_WORK_TREE=$HOME/git.git
    GIT_DIR=$GIT_WORK_TREE/.git
    export GIT_WORK_TREE GIT_DIR
    cd /tmp
    git pull

At the beginning of git-pull, we check "require-work-tree" and then
"cd-to-toplevel".  I _think_ the original intention when I wrote the
command was "we MUST have a work tree, our $(cwd) might not be at the
top-level directory of it", and no stronger than that.  That check is a
very sensible thing to do before doing cd-to-toplevel.  We check that the
place we would want to go exists, and then go there.

But the implementation of require_work_tree we have today is quite
different.  I don't have energy to dig the history, but currently it says:

    test "$(git rev-parse --is-inside-work-tree 2>/dev/null)" = true ||
    die "fatal: $0 cannot be used without a working tree."

Which is completely bogus.  Even though we may happen to be just outside
of it right now, we may have a working tree that we can cd_to_toplevel
back to.

Add a function "require_work_tree_exists" that implements the check
this function originally intended (this is so that third-party scripts
that rely on the current behaviour do not have to get broken).

For now, update _no_ in-tree scripts, not even "git pull", as nobody on
the list seems to really care about the above corner case workflow that
triggered this. Scripts can be updated after vetting that they do want the
"we want to make sure the place we are going to go actually exists"
semantics.

Signed-off-by: Junio C Hamano <gitster@pobox.com>