feat: updating pending and wip logic

[bethesque]

Current logic for determining the WIP pacts list ends with:

5. Discard all the pacts that were published before the first instance of this provider tag (this is so that if you create a brand new provider branch, you don't get EVERY head pact included in the WIP pacts list).

The problem with this is that feature pacts created before a provider's feature branch is created don't get included.

See #427 and https://github.com/pact-foundation/pact_broker/blob/a59f19b27f1d01a48ff62e55b81e9c27cbdde2fe/script/reproduce-issue-starting-up.rb

This PR changes the last condition to:

5. Discard all the pacts that were explicitly selected and successfully verified by another branch of the provider before this branch was created (this is so that if you create a brand new provider branch, you don't get EVERY head pact included in the WIP pacts list).

[bethesque]

It also changes the pending logic from:

A pact content is considered pending if there has not been a successful published verification by the specified branch of the provider.

to

A pact content is considered pending if there has not been a successful published verification by the specified branch of the provider, and there wasn't a successful published verification from a different branch from before this branch was created.

[solarmosaic-kflorence]

We should add this to the list of things to update in the WIP/pending docs: pact-foundation/docs.pact.io#26