upgrade: extract the checking of target requirements change This logic is fairly independant, lets move it out of the main function. Differential Revision: https://phab.mercurial-scm.org/D9485

upgrade: drop an outdated comment We can control the added/removed requirement through config for multiple years. Differential Revision: https://phab.mercurial-scm.org/D9484

upgrade: gather code about source checking together They just moved in the same file, now they are grouped together and documented. Differential Revision: https://phab.mercurial-scm.org/D9483

upgrade: move requirements checking in a dedicated function This is a simple an isolated check that can go next to the associated code. Differential Revision: https://phab.mercurial-scm.org/D9482

upgrade: split definition and management of the actions from the main code This is a second step to clarify and clean up this code. The code responsible for definition which action exist, are possible and their compatibility if moved into a sub module. This clarify the main code and prepare further cleanup. Differential Revision: https://phab.mercurial-scm.org/D9477

upgrade: split actual upgrade code away from the main module The main module is getting big and hard to follow. So we are splitting all the logic to actually run an upgrade in a sub module. It nicely highlight that there are very few actual call point to the code we just moved. Differential Revision: https://phab.mercurial-scm.org/D9476

phab-refresh: add an explanatory message This serve two purposes: - clarify why a bot just touched this diff, - explicit the fact the tests have been run, and are passing on this version. Differential Revision: https://phab.mercurial-scm.org/D9527