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

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>