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.

[tox]
minversion = 1.8
envlist = py{26,27}-{pytest,nose}

[testenv]
setenv =
    PYTHONHASHSEED = 0
deps =
    nose: nose
    pytest: pytest
commands =
    nose: nosetests {posargs}
    pytest: py.test {posargs}