templates: define a {onelinesummary} keyword It is sometimes useful to be able to use the configured `command-template.oneline-summary` in higher-level templates. For example, I would like to use it in an internal template that lists commits in a "review unit" (kind of a pull request). This patch adds support for that. We may want to define a way of formatting a context using a command-specific override (from `command-templates.oneline-summary.<command>`), but that will have to be a template function instead. I don't plan to do that, but I'm mentioning it now in case reviewers would prefer that we use a no-arg function (i.e. `{onelinesummary()}`) already today to prepare for that. Differential Revision: https://phab.mercurial-scm.org/D9314