l10n: de.po: translate 4 new messages

Translate 4 new messages came from git.pot update in 28e1aaa4 (l10n:
git.pot: v2.13.0 round 2 (4 new, 7 removed)).
Signed-off-by: NRalf Thielow <ralf.thielow@gmail.com>
Acked-by: NMatthias Rüster <matthias.ruester@gmail.com>