Cow does not include new modules in the changelog for their first release

[GuySartorelli]

If we add a new module (e.g. if we add support for a module we previously didn't support) we currently have to add a one-off hard-coded hack to CreateChangelog.php's getChangelogLibrary() method. Otherwise, cow will skip generating the changelog for any module that wasn't in the recipe's previous release.
This caused silverstripe/developer-docs to not be included initially for the 4.12 release, which was the first time it was being included by cow.

Acceptance criteria

• Cow automatically correctly handles modules the first time they are added to a cow release
• For modules which have had prior versions which were not supported, the most recent tag prior to work is used as the previous version for generating the changelog
• For modules which had no prior version at all, one of the following is used:
  • If the code was copied from another module (e.g. silverstripe/template-engine was copied from silverstripe/framework originally) the changelog shows all commits that are unique to the new repo (e.g. don't show framework commits in the docs against the template engine repo).
  • If the code is entirely new, all commits in the repo are shown in the changelog