Git merge driver for ChangeLog files.

1. It produces no conflict when ChangeLog entries have been inserted
at the top both in the public and in the private modification. It
puts the privately added entries above the publicly added entries.

2. It respects the structure of ChangeLog files: entries are not split
into lines but kept together.

3. It also handles the case of small modifications of past ChangeLog
entries, or of removed ChangeLog entries: they are merged as one
would expect it.

4. Conflicts are presented at the top of the file, rather than where
they occurred, so that the user will see them immediately. (Unlike
for source code written in some programming language, conflict markers
that are located several hundreds lines from the top will not cause
any syntax error and therefore would be likely to remain unnoticed.)