CLI: generate-changelog from arbitrary commits

[nphilipp]

Story

As someone resolving an unresolvable merge,
I want to be able to generate changelogs from arbitrary commits,
so stitching together a sensible changelog history is less cumbersome.

Acceptance Criteria

• rpmautospec generate-changelog accepts a parameter to generate changelogs from arbitrary commits

Background

If a merge happens in a package repository, right now users have to checkout the parent commits manually, generate the changelog for them and manually merge. This would make repairing changelogs after merges a bit less cumbersome.