gen-release-notes: prettify only lowercased headers

Now one can write either `## bugfix/luajit` or `## bugfix/LuaJIT`. The
latter will NOT be transformed into `### Luajit` anymore. Both variants
now give `### LuaJIT` section inside `## Bugs fixed` section.

See `REDEFINITIONS` variable inside the script to understand how well
known headers (such as `## <...>/LuaJIT`) are prettified.

There is a problem with section grouping, when headers are written in
lower/title/mixed case. It'll be resolved in a next commit.

(cherry picked from commit df5d69e6)