l10n: de.po: translate 68 new messages

Translate 68 new messages came from git.pot update in 727b957
(l10n: git.pot: v1.8.5 round 1 (68 new, 9 removed)).

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
l10n: de.po: improve error message when pushing to unknown upstream

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