Provide a way to include the generated changelog file in the release

To do a release with Gren, the workflow is:
- put a tag on a commit
- run `gren release`

After this is done, if one wants a textual `CHANGELOG.md` file, one can run `gren changelog`, which retrieves the description of the release from the repo and puts it into a file.

Then, if we want this generated `CHANGELOG.md` to be part of the repo, we add it with a commit. However, this means this file is not part of the release tarball we just made, since this new commit will come after the tag.

Could we imagine a way to be able to make a release that would include its own updated `CHANGELOG.md` file?

Start by tracing the `gren release` and `gren changelog` entry points and how the release tarball is created. Done means a release can include its generated `CHANGELOG.md` in the tarball without requiring a later commit.

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