git-gui: set GIT_DIR and GIT_WORK_TREE after setup

git-gui: set GIT_DIR and GIT_WORK_TREE after setup

Rather than juggling with the env var GIT_DIR around the invocation of
gitk, set it and GIT_WORK_TREE after finishing setup, ensuring that any
external tool works with the setup we're running with.

This also allows us to remove a couple of conditionals when running gitk
or git gui in a submodule, as we know that the variables are present and
have to be unset and reset before and after the invocation.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>