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 <6010d7f2c049984f25cd19568d4ace2cc46b0817@gmail.com>
Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git