`MergeRequest#rebase_async` can lock indefinitely with a SELECT FOR
UPDATE call. If an update is idle in transaction or another rebase
attempts to run, `MergeRequest#rebase_async` could queue indefinitely or
until statement timeouts are triggered.

To limit the impact of this operation, we now just bail out after 5 s if
we can't get the lock. The user will see an error message if this
happens.

Part of #30528