rebase: merging indirectly via combined branch does not rebase higher prs

BUG:

We create release branches that combine many PRs/branches into a single PR. When that PR is merged to the root the original PRs are closed as merged, BUT any stacked PRs do not rebase onto the root.

Repro:

1. Create three PRs A and B and C, with B stacked on A.
2. Create a third PR, D, which combines branches A and C.
3. Merge D onto master.
4. Observe that B is still stacked on A, and if branch A is deleted, PR B is auto closed.

No file, test, or entry point is named. Reproduce the A/B/C and combined D scenario, then trace the handling of merged pull requests and stacked-branch rebases; the fix is complete when B is rebased onto master after D merges and is not auto-closed when A is deleted.

Written by the indexing model from the issue text.