Correctly format date in mail headers

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Include CC only if necessary

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Include Message-ID in email

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Dropped unused code

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Deduplicate code

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Fixed issue #1839: Conflict dialog does not allow resolving multiple or all conflicts with the same option

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