Vundle manipulates plugin folders by `cd`-ing into them and calling git
via `call system(cmd)`. This doesn't work if GIT_DIR is set as a global
environmental variable (most commonly by a git hook). Since there is no
use case in which Vundle should respect GIT_DIR, this PR preemptively
precedes all git calls with `unset GIT_DIR; `. This makes the git calls
behave as expected without unsetting GIT_DIR in the environment beyond
vim.