If an automatic backport fails to more than one release branch we need
to crate a PR to backport it. So far we've had to create a backport PR
for each target release branch.
With this change, we may be able to create only a single PR, and then
run the backport automation to cherry-pick it into other release
branches.
The idea is that if a change introduced in version n-1 caused a
conflict, and there are no other changes, then the backport automation
should be able to use the same commit for version n-2 and n-3.
1f2531a664