Check that no line from any of the input files was lost.
This is not perfect for several reasons.
It doesn't check that the content goes to the desired location.
It doesn't check that sections are created as necessary.
It doesn't support whitespace normalization that the parsing code does.
But it's a good start.
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.
