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"

The difference between "getonefile" and "cat" command is:
"getonefile" gets the CORRECT EOL which is identical with the working tree,
and "cat" gets the ORIGINAL EOL stored in repository.

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)
Using "getonefile" command to get CORRECT EOL which is identical with the working tree,
instead of using "cat" command to get the ORIGINAL EOL which is not filtered and might be wrong.

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