upgrade: move optimisation to something more declarative
This is not great yet, but still better than the previous state. This get use
one step closer to having all the possible "actions" clearly declared and moved
in a dedicated module.
Differential Revision: https://phab.mercurial-scm.org/D9475
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.