Sync TortoiseIDiff with TortoiseSVN

Based on TortoiseSVN revision 24470

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Implement 3-pane TortoiseGitIDiff

Based on TortoiseSVN revision 24640, 24644, 24645, 24753

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Can call TortoiseIDiff to resolve image conflict

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Also handle .tif and .tiff to call TortoiseGitIDiff

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
CAppUtils::StartExtMerge follows the style of TortoiseSVN

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