api breakage tester: fetch all refs (#1076)
api breakage tester: fetch all refs (#1076)

Motivation:

To be able to use SHA sums as git versions, we need to pull all the refs
to we get even the commit SHAs from PRs. That's important so CI can use
this script.

Modifications:

Fetch `+refs/pull/*:refs/remotes/origin/pr/*` refs.

Result:

Should be able to be usable in CI across forks.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio