l10n: de.po: translate 28 new messages

Translate 28 new messages came from git.pot update in
df49095 (l10n: git.pot: v1.9 round 1 (27 new, 11 removed)
and d57b24b (l10n: git.pot: v1.9 round 2 (1 new)).

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>