l10n: de.po: correct message when hiding commits by craft

The recent translation was giving the idea that all commits
based on a graft were meant to be hidden. Make it clear that
it is the graft commit itself.
Reported-by: NDavid Kastrup <dak@gnu.org>
Signed-off-by: NRalf Thielow <ralf.thielow@gmail.com>

po/de.po

@@ -7878,7 +7878,7 @@ msgstr "Komprimierungsgrad für Paketierung"
 
 #: builtin/pack-objects.c:2506
 msgid "do not hide commits by grafts"
-msgstr "verbirgt keine Commits mit künstlichen Vorgängern (\"grafts\")"
+msgstr "verbirgt keine künstlichen Vorgänger-Commits (\"grafts\")"
 
 #: builtin/pack-refs.c:6
 msgid "git pack-refs [options]"