update the german translation file header

Include the previous translators in the header and update some of the placeholders.

Signed-off-by: Matthias Aßhauer <mha1993@live.de>

po/documentation.de.po

-# SOME DESCRIPTIVE TITLE
+# German translations for Git Manual Pages.
 # Copyright (C) YEAR Free Software Foundation, Inc.
-# This file is distributed under the same license as the PACKAGE package.
-# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
-#
-#
+# This file is distributed under the same license as the Git package.
+# Fabio Scotoni <fabio@esse.ch>, 2008.
+# Mark Struberg <struberg@yahoo.de>, 2008
+# Jean-Noël Avila <jn.avila@free.fr>, 2019.
+# 
 msgid ""
 msgstr ""
-"Project-Id-Version: PACKAGE VERSION\n"
+"Project-Id-Version: git documentation\n"
 "POT-Creation-Date: 2019-01-04 23:29+0100\n"
-"PO-Revision-Date: 2019-01-04 21:54+0100\n"
-"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"PO-Revision-Date: 2019-01-09 20:52+0100\n"
+"Last-Translator: Jean-Noël Avila <jn.avila@free.fr>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
-"Language: \n"
+"Language: de\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"