Update libgit2

LF -> CRLF filter refuses to handle mixed-EOL files

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Extend first argument of CGit::GetOneFile() to get filtered content from blob directly

The original CGit::GetOneFile() is designed for commit only.
From now on, support blob also.

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

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Add description of new command "getonefile" and mark "cat" command as "Deprecate"

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

(Fix issue #2003)

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