git-gui: translate the remaining messages in zh_cn.po to chinese

'make' shows:
  MSGFMT po/zh_cn.msg 368 translated, 2 fuzzy, 1 untranslated message.

1. update the zh_cn.po and translate the remaining messages in chinese

2. correct some of the previously mis-translated messages

3. add a list of word interpretation in the head as a guideline for
   subsequent updatings and translations

Signed-off-by: eric miao <eric.miao@marvell.com>
Acked-by: Xudong Guan <xudong.guan@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>