Fix wrong whitespace marker position if has tabs or characters of wider width

Based on TortoiseSVN revision 25583

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Broken characters when dragging the horizontal bar in TortoiseGitMerge

Based on TortoiseSVN revision 25584

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Correct click and drag cursor position for wider characters

Based on TortoiseSVN revision 25585

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