Look for $GIT_DIR and $GIT_WORK_TREE

There are rare situations where a user has manually specified what they
wish to use as their work-tree directory, and even rarer situations
where the user wishes the Git directory to be customized. In the case
the user has set these using environment variables, vim-fugitive takes
advantage of these settings in order to set up.

Note that git-config(1) allows setting the work-tree and Git dir in
a number of ways (see the core.worktree) setting. This change only
respects the environment variable method, not the config file method.