Changes get automatically added to changelogs in reverse chronological order when it should be forward order

When users adds a change to a changelog manually, we can reasonably expect that if there is a section already present they will add the new entry to the end of the list rather than the beginning.

However, when `auto-changelog` copies commit messages into the Uncategorized section of a changelog, it does so in reverse chronological order, effectively always prepending to the beginning of the list.

This is likely to cause some confusion when we change the process to ask people to update changelogs while they make changes. Users are likely not to notice that Uncategorized changes are reversed until they try to consolidate similar changes (say, successive version bumps to the same package). And even if they do, they will need to move entries from Uncategorized to the relevant categories in reverse order for everything to be consistent.

## Acceptance Criteria

- The tool copies commit messages to Uncategorized in forward chronological order (earliest to latest).