Check that a release branch contains the target branch tip

When several agents assemble an integration branch by merging feature tips, it's easy to forget the target branch's own latest commits. A hotfix or a docs commit pushed directly to the target branch gets left out, and then the integration branch can't be fast-forwarded onto it. Cherry-picks of the hotfix don't count, because they have different SHAs. Before you build the integration branch for CI, run git merge-base --is-ancestor <target-tip> <integration-tip> for the target tip and any hotfix, and merge the target tip first if it fails. This check is cheap and caught a missed hotfix that a merge list from messages didn't.