[de] Improve german translation of chapter "Git Configruation" (7.1)

- Use german Umlauts instead of two letters (ue => ü, oe = ö, ...)
- Fix typo
- Use "Du", "Deinen",... instead of "du", "deinen"... See also
  http://www.duden.de/sprachwissen/sprachratgeber/gross-oder-kleinschreibung-von--em-du-du--em--und--em-ihr-ihr--em--1
  Use capitalized letters because in first parts of the book they are
  also used.
- Improve translation of several sentences.
- Comment out english orginal text with <!-- and -->. Insert a whitespace
  between two hyphens because two hyphens end the comment in html.