contrib/backporting: Fix commit order in check-stable
contrib/backporting: Fix commit order in check-stable

For particular PRs, for example #7476, the chronoligcal ordering of
commits based upon AuthorDate is not the same as the revision list
ordering in git, which caused the list of commits to be out-of-order
when printing on the commandline. This could cause problems during
backporting where backporters would inadvertently attempt to backport
later commits before earlier commits that they depend on, leading to
more conflicts than necessary during the backport process.

Fix this up by using the canonical commit ordering in the branch that
was merged, and only use GitHub's PR commit list as a way to determine
the total number of commits being merged in the PR.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

cilium/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,cilium-team/cilium,cilium-team/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium