TortoiseGitMerge: More whitespace-fix handling

use a separate dialog for the setup: reusing the same dialog should be avoided

don't treat labels as buttons

never automatically fix: if there is something to fix, always ask first

use defines instead of hard coded values

use two-state checkboxes, not tri-state in setup mode

Based on TortoiseSVN revision 24358, 24360, 24361

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