git-submodule: work with GIT_DIR/GIT_WORK_TREE

git-submodule: work with GIT_DIR/GIT_WORK_TREE

The combination of GIT_DIR and GIT_WORK_TREE can be used to manage
files in one directory hierarchy while keeping the repository that
keeps track of them outside the directory hierarchy.  For example:

    git init --bare /path/to/there
    alias dotfiles="GIT_DIR=/path/to/there GIT_WORK_TREE=/path/to/here git"

    cd /path/to/here
    dotfiles add file
    dotfiles commit -a -m "add /path/to/here/file"
    ...

lets you manage files under /path/to/here/ in the repository located
at /path/to/there.

git-submodule however fails to add submodules, as it is confused by
GIT_DIR and GIT_WORK_TREE environment variables when it tries to
work in the submodule, like so:

    dotfiles submodule add http://path.to/submodule
    fatal: working tree '/path/to/here' already exists.

Simply unsetting the environment where the command works on the
submodule is sufficient to fix this, as it has set things up so
that GIT_DIR and GIT_WORK_TREE do not even have to point at the
repository and the working tree of the submodule.

Signed-off-by: Daniel Graña <dangra@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>