Add special GIT_CMD_ADD entry for using libgit2 for adding files

This is added so that people using clean filters which do not seem to be supported by libgit2 do not need to disable libgit2 completely.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Update comments for not using libgit2 in CommitDlg

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Use more CGit::CombinePath() to refactor code

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>