Add a rudimentary script to generate release changelog.
Add a rudimentary script to generate release changelog.

The idea is very simple: use the list in the release branch to know
which changelog entries to include, but do the work of removing these
entries and consolidating the released changelog in the master branch
(so that it is applied both to the master branch and to the release
branch following the backporting process).

ejgallego/coq,SkySkimmer/coq,gares/coq,coq/coq,silene/coq,JasonGross/coq,SkySkimmer/coq,ejgallego/coq,coq/coq,Zimmi48/coq,ejgallego/coq,JasonGross/coq,silene/coq,Zimmi48/coq,gares/coq,coq/coq,ejgallego/coq,gares/coq,gares/coq,SkySkimmer/coq,SkySkimmer/coq,JasonGross/coq,Zimmi48/coq,silene/coq,gares/coq,coq/coq,JasonGross/coq,Zimmi48/coq