Add global-revisions flag

[felixjung]

I'm trying to include openapi-changes in our GitHub Actions CI to detect breaking changes in pull requests. My approach to doing this was to take the number of commits made on a branch n (given by github.event.pull_request.commits) and passing n+1 to openapi-changes via the --limit flag.

Problem
What I found was that the tool looks at the full local git history of the file, and uses the --limit flag to cut off after n commits. This leads to problems when a PR has 10 commits, but none or only some changed the OpenAPI spec being checked.

Approach
As a quick fix, this PR adds a global-revisions boolean flag, which is used to instruct git to list the history between specific revisions (i.e., HEAD~(n+1)..HEAD).

Alternative Approaches
I've seen #85 and I think the approach of passing git revisions directly makes a log of sense. However, I think there's some ambiguity around how this should be implemented.

• One could pass two revisions and do a left-right comparison.
• One could pass two revisions and do a report for every commit between them, as is the current behavior.
• One could take a full revisions argument string that matches the supported inputs of git log itself.

All these are worth exploring, but I needed a quick win.