l10n: zh_CN.po: translate 27 messages (2210t0f0u)

Translations for git v1.9-rc0, and also update translations on "graft"
and "reference repository".

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
l10n: remove 2 blank translations on Danish, Dutch

Two l10n teams haven't contributed a single translation for about two
years since they was initialized with a blank template.  Remove them
can make the Git package smaller and give opportunities to other
contributors.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>