This also implements support for commits as change note references.
They are rendered separately. When there are several change note
fragment files symlinked, with numbers and commit hashes pointing to
the same changelog category, all of them are enumerated. PRs/issue
numbers a listed in a paragraph that is separate from the commit
links.

Specifically, the new Towncrier fragment types are set up to be
`bugfix`, `feature`, `deprecation`, `breaking`, `doc`, `packaging`,
`contrib`, `misc`.

Previously, the `misc` fragments were rendered without the entry
context, just as a list of PR numbers. With this change, the change
note text is displayed as usual.