l10n: Review zh_CN translation for Git 1.7.10-rc1

Overall review of the zh_CN translation:

 - Distinguish the translations of index and stage, though they are the
   same thing.

 - Many other fixes, e.g., add the lost periods at the end of translated
   sentences.
Signed-off-by: NJiang Xin <worldhello.net@gmail.com>