Fix cancelling a rebase
When clicking on the rebase button you are prompted with a dialog to
confirm if you want the rebase or not.
Turns out we were not respecting the user's choice if they cancelled
there.
With this commit, we do.
Fixes https://pagure.io/pagure/issue/4267
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>