Don't set GIT_DIR when staging git clones

GIT_DIR is convenient, but it does not work when both a formula and
subformula try to stage a git checkout, as git will use the main
formula's repository when staging the subformula.

Stop setting it, and restore the kludge to the git-multipush formula.

Fixes Homebrew/homebrew#10893.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>