Add command to create a full changelog from previous releases

There could be cases when you want to take the set of partial changelogs and generate a full changelog from all previous releases. A command that is capable of doing this without having to recreate the changelog from the set of PRs and issues would be ideal --especially if there were manual changes made to the changelog (which is common).

Start by tracing the existing changelog-generation command and how partial changelogs from previous releases are represented. Define the command's input and output behavior around manually edited changelogs, then verify that it combines all prior release changelogs without rebuilding them from PRs and issues.

Written by the indexing model from the issue text.