retry does not work when treeclosed

[kennytm]

(Tried on rust-lang/rust's bors, not sure how updated is that)

Repro steps:

1. Close the tree with treeclosed=1
2. Let all pending PRs finish.
3. Select a failed PR and give it p=2.
4. Send a retry

Expected:

• Homu should create a merge commit and move the PR to "pending" state

Actual:

• The PR's state becomes "approved", but nothing else happens.

Note: The PR will start immediately after issuing a treeclosed-.