l10n: zh_CN: Add translations for Git glossary

Add translations of Git glossary (most of them are from the command `git
help glossary`) in the header of `zh_CN.po`.  Also fixes some translations
according to this glossary, such as "pathspec", "repository", "refspec".

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