Currently, once a source PR has been merged it's not possible to set
or update a limit, which can be inconvenient (e.g. people might have
forgotten to set it, or realise after the fact that setting one is not
useful, or might realise after the fact that they should *unset* it).
This PR relaxes that constraint (which is not just a relaxation as it
requires a bunch of additional work and validation), it should now be
possible to:
- update the limit to any target, as long as that target is at or
above the current forwardport tip's
- with the exception of a tip being forward ported, as that can't be
cancelled
- resume a forward port stopped by a previously set limit (just
increase it to whatever)
- set a limit from any forward-port PR
- set a limit from a source, post-merge
The process should also provide pretty chatty feedback:
- feedback on the source and closest root
- feedback about adjustments if any (e.g. setting the limit to B but
there's already a forward port to C, the 'bot should set the limit
to C and tell you that it happened and why)
Fixes#506
