l10n: de.po: translate 28 new messages

Translate 28 new messages came from git.pot update in
df49095a (l10n: git.pot: v1.9 round 1 (27 new, 11 removed)
and d57b24b6 (l10n: git.pot: v1.9 round 2 (1 new)).
Signed-off-by: NRalf Thielow <ralf.thielow@gmail.com>