l10n: de.po: translate "index" as "Index"

The term "index" is translated as "Staging-Area" to
match a majority of German books and to not confuse
Git beginners who don't know about Git's index.

"Staging Area" is used in German books as a thing where
content can be staged for commit.  While the translation
is good for those kind of messages, it's bad for messages
that mean the Git index as the tree state or the index
file, in which case we should translate as "Index".

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