Import of new `diffrelease` script
This replaces both the old CVS diffrelease script and the rel2html tool
which further munged its output, and should produce reasonably similar output
to what they did. Hypertext links are now restructured to follow URLs exposed
by GitLab rather than by the CVS viewer. Regressed commits are now included
but with strikeout. Language in the HTML headers is set to English rather than
German(!)
Tested on Linux and Windows.