git-gui: remove unnecessary 'cd $_gitworktree' from do_gitk

In the procedure that invokes Gitk, we have a 'cd $_gitworktree'. Such a change of the current directory is not necessary, because - if we have a working tree, then the startup routine has already changed the current directory to the root of the working tree, which *is* $_gitworktree; or - if we are in a bare repository, then there is no point in changing the current directory anywhere. (And $_gitworktree is empty.) Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Mark Levedahl <mlevedahl@gmail.com> Signed-off-by: Johannes Sixt <j6t@kdbg.org>