Merging a PR when another PR based on it is opened causes weirdness

[lleger]

Given:
PR #1 is open
PR #2 is open and based on PR #1

When:
PR #1 is merged, Custodian deletes its branch, causing PR #2 to be closed, delabeled, and have its branch deleted.

This is a bug, since PR #2 should still remain open.

Possible fix would be to check for open PRs based on a branch before deleting it. If found, rebase those branches to master instead before deleting. I think the would solve the issue.