formatter: add support for changeset templating Some formatter-based commands provide fields that are identical to the ones defined in templatekw, but we had to specify them manually to support all changeset-based template keywords. This patch adds fm.context() that populates all templatekw. These keywords are available only in template output, so we still need to set important keywords via fm.data() if they should be available in e.g. JSON output. Currently fm.context() takes only 'ctx' argument. It will eventually be extended to take 'fctx' to support file-based keywords (e.g. {path}) seen in hgweb.