fix: refactor getrevstofix() to define revisions first, then validate them
This refactoring makes it easier to add a new way of specifying
revisions (I'm about to add a `--source`, which adds the specified
revisions and their descendants).
Differential Revision: https://phab.mercurial-scm.org/D8285
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}