changelog: rework changeset selection and button behaviour This is a refactoring / rework of messy code to be simpler and work correctly. As a result, it gives slightly different behaviour in some cases and different button labels. It also allows creating PRs after selecting two changesets. That is currently pointless but will make sense soon.

syntax: glob
*.pyc
*.swp
*.sqlite
*.tox
*.egg-info
*.egg
*.mo
.eggs/
tarballcache/

syntax: regexp
^rcextensions
^build
^dist/
^docs/build/
^docs/_build/
^data$
^kallithea/tests/data$
^sql_dumps/
^\.settings$
^\.project$
^\.pydevproject$
^\.coverage$
^kallithea\.db$
^test\.db$
^Kallithea\.egg-info$
^my\.ini$
^fabfile.py
^\.idea$