Don't set $GIT_DIR and alike

Setting environment variables like $GIT_DIR and $GIT_WORK_TREE for an emacs instance is a recipe for disaster.

The problem is that environment variables are shared across all of Emacs, so every buffer shares the same value. The equivalent outside outside of Emacs would be if doing export VAR=VAL in one terminal would take effect all other current and future terminals.

This means that certain command-line tools such as vcsh don’t work properly in combination with Emacs/Magit. See #2939.

Unfortunately git commit does export $GIT_DIR before calling $EDITOR, leading to issues such as #3516, so Magit now unsets that variable and $GIT_WORK_TREE if they are set when magit is loaded.