git-gui: Protect ourselves from funny GIT_DIR/working directory setups.

git-gui: Protect ourselves from funny GIT_DIR/working directory setups.

Since we have some serious problems with the GIT_DIR environment variable
on Windows we cannot let the user use a non-standard GIT_DIR with their
working directory.

So require that the GIT_DIR name is actually ".git", that it exists,
and that its parent directory is our working directory.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>