RebaseDlg: Allow to split commits

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Do not disable Amend if error occurred

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Disable Amend if cherry pick is active is

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Merge branch 'rebase-split-commits'
Fixed issue #2311: Empty commits cannot be rebased

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