Deduplicate identical changesets

[blast-hardcheese]

As groups of dependencies managed by the same constant or variable end up looking independently updatable, we invariably end up with duplicate PRs with the same content.

Example:

• Update jackson-annotations, jackson-core to 2.12.2 guardrail-dev/guardrail#1012
• Update jackson-databind to 2.12.2 guardrail-dev/guardrail#1013
• Update jackson-datatype-jsr310 to 2.12.2 guardrail-dev/guardrail#1014
• Update jackson-module-scala to 2.12.2 guardrail-dev/guardrail#1015

I'm not sure how best to resolve this, a naive approach would be to order by GAV then deduplicate via a sliding window, we'd only get the first "group" opened and subsequent runs would deduplicate. This presumes common prefix, but that may be a good starting place.