MergeDialog: codefix.
MergeDialog: small codefix.
MergeDialog: source_target_edit is added.
MergeDialog: Merge options is added.
MergeDialog: DialogButtonBox is added.
Git: merge is added.
MergeDialog: merge is realised.
MergeDialog: disable fast_forward_option when squash checked.
Git: abort_merge is added.
MainWindow: Abort merge is added.