Report template changes in sync PRs

[simu]

Context

Currently, commodore component sync and commodore package sync generate PR descriptions which only contain the commit hash of the new template version which is getting applied. It would be nice if the PR description also contained a list of all commits (or a link to the template diff) which were used to generate the changes presented in the PR.

Acceptance criteria

• Commodore renders the template changes as a link to the template diff on GitHub in the sync PR description

Implementation ideas

This will require some additional logic in the dependency templater, as we may be applying different sets of changes for different dependencies. We most likely want to extract the old template commit id from .cruft.json before the update and the new one afterwards. We'll also need to clone the template repo to extract the commit messages for all commits between the two commit ids.

Ideally we'll omit merge commits to reduce the list to only relevant commits.

Alternatively, we could maybe use PR labels (assuming the template is hosted on GitHub) to generate a changelog based on merged PRs since the last template update.