sanitize-commit: drop "not a valid branch" from -2 to -1

... making it overridable for reviewers.

This regularly triggers in qt5compat, where Pick-to: 5.15 means qtbase
(and the cherry-pick dashboard accurately tracks across modules),
causing authors to drop the 5.15 pick-to to get the change past the
bot's -2, therefore lying to reviewers and the cherry-pick dashboard
alike).

We're adults, we don't need a parental unoverridable -2.

Change-Id: I78c4ff7596d8ba91152ded09672105064f1ec426
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Daniel Smith <daniel.smith@qt.io>