git-gui: Handle failure of core.worktree to identify the working directory.

git-gui: Handle failure of core.worktree to identify the working directory.

Commit 21985a11 'git-gui: handle non-standard worktree locations' attempts
to use either GIT_WORK_TREE or core.worktree to set the _gitworktree
variable but these may not be set which leads to a failure to launch
gitk to review history. Use _gitdir to set the location for a standard
git layout where the parent of the .git directory is the working tree.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>