Support left and right comparison between commits

[jackfrankland]

Hi!

There are two use cases that I'd love this tool to cater for:

1. Before committing changes, a check to see I have not introduced breaking changes to the api spec, either against head or to another branch (i.e. main)
2. The CI pipeline to detect breaking changes between the head and the base branch on PR

I believe these could potentially be achieved by allowing to optionally specify the git commit/reference:

openapi-changes <revision>:openapi.yaml ./openapi.yaml