hooks: refactor log_push_action

The core of the functionality is to process a list of "raw id"s, log them, and
update / invalidate caches.

handle_git_post_receive and scm _handle_push already provide that list
directly. Things get much simpler when introducing a new function
(process_pushed_raw_ids) just for processing pushed raw ids. That also makes it
clear that scm _handle_push doesn't need any repo.

log_push_action remains the native entry point for the Mercurial hook. It was
not entirely correct using 'node:tip' - after Mercurial 3.7 and d6d3cf5fda6f,
it should be 'node:node_last'.

After several trivial refactorings, it turns out that the logic for creating
the hash list for Mercurial actually is very simple ...