git-gui: update German translation

Update German translation (glossary and final translation) with
recent additions, but also switch several terms from uncommon
translations back to English vocabulary.

This most prominently concerns "commit" (noun, verb), "repository",
"branch", and some more. These uncommon translations have been introduced
long ago and never been changed since. In fact, the whole German
translation here hasn't been touched for a long time. However, in German
literature and magazines, git-gui is regularly noted for its uncommon
choice of translated vocabulary. This somewhat distracts from the actual
benefits of this tool. So it is probably better to abandon the uncommon
translations and rather stick to the common English vocabulary in git
version control.

Signed-off-by: Christian Stimming <[email protected]>

Author: cstim