vim-fugitive now works correctly with vcsh

[John Whitley]

My PR for tpope/vim-fugitive#415 [1] on GitHub was just accepted, which enhances vim-fugitive to work with vcsh.  More specifically, fugitive now works correctly when $GIT_DIR is set ,but core.worktree is set instead of $GIT_WORK_TREE.

Vim users, after updating to the latest vim-fugitive, the following now works:

>> vcsh <some repo>
>> vi
# :Gwrite and other fugitive commands are now defined and work

Cheers,
John

[1] https://github.com/tpope/vim-fugitive/issues/415