Update libgit2

Need this to fix issue #2017

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Extend first argument of CGit::GetOneFile() to handle with blob refname

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Add GetOneFileCommand

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Use command "getonefile" to get Versioned file, instead of "cat" command

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>