[CI] Changelog should compare to common ancestor

[djaglowski]

Similar issue to #11442.

See #12124(comment)

[djaglowski]

I was able to reproduce the issue locally by running the check commands against a branch that was created prior to the CHANGELOG.md being updated on main.

➜  opentelemetry-collector-contrib (untangle-file-attributes) git fetch origin main --depth=1
remote: Total 0 (delta 0), reused 0 (delta 0), pack-reused 0
From https://github.com/open-telemetry/opentelemetry-collector-contrib
 * branch                  main       -> FETCH_HEAD
➜  opentelemetry-collector-contrib (untangle-file-attributes) git diff --name-only FETCH_HEAD                        
CHANGELOG.md
CONTRIBUTING.md
cmd/configschema/go.mod
cmd/configschema/go.sum
cmd/mdatagen/go.mod
cmd/mdatagen/go.sum
exporter/alibabacloudlogserviceexporter/go.mod
exporter/alibabacloudlogserviceexporter/go.sum
...
➜  opentelemetry-collector-contrib (untangle-file-attributes) git diff --name-only $(git merge-base origin/main HEAD)