assemble_changelog.py reads changelog entries from ChangeLog.d/*.md
and merges them into ChangeLog.md.
The changelog entries are merged into the first version in
ChangeLog.md. The script does not yet support creating a new version
in ChangeLog.md.
The changelog entries are merged in alphabetical order of the file
names. Future versions of this script are likely to adopt a different
order that reflects the git history of the entries.
