If available, use gitk --argscmd in contrib/stg-gitk.
This allows to ask gitk to recompute the list of patches to show at
every refresh. Before this, we had problems with 'stg-gitk --all':
- deleting a patch that was existing at startup time would trigger an
"unknown ref" error from gitk and force to quit/restart manually;
- patches created since startup were only visible when applied, or
when below one of the startup patches.
Note that --argscmd is not in official gitk yet, so we don't try to
use it if it's not available.
Signed-off-by: Yann Dirson <ydirson@altern.org>