json: implement {helptopics} template

json: implement {manifest} template Property naming was borrowed from `hg files -Tjson`. We omit branch because, again, representation of branches in this template is wonky.

json: implement {shortlog} and {changelog} templates These are the same dispatch function under the hood. The only difference is the default number of entries to render and the template to use. So it makes sense to use a shared template. Format for {changelistentry} is similar to {changeset}. However, there are differences to argument names and their values preventing us from (easily) using the same template. (Perhaps there is room to consolidate the templates as a follow-up.) We're currently not recording some data in {changelistentry} that exists in {changeset}. This includes the branch name. This should be added in a follow-up. For now, something is better than nothing.

help: populate template functions via docstrings We do this for revsets, template keywrods, and template filters. Now we do it for template functions as well.

templater: add consistent docstrings to functions The content of "hg help templating" is largely derived from docstrings on functions providing functionality. Template functions are the long holdout. Prepare for generating them dynamically by defining docstrings for all template functions. There are numerous ways these docs could be improved. Right now, the help output simply shows function names and arguments. So literally any accurate data is better than what is there now.

changeset_printer: hide manifest node for workingctx Because workingctx has no manifest, it makes sense to hide "manifest:" row completely.

changeset_printer: display p1rev:p1node with "+" suffix for workingctx Still templater can't handle workingctx, which will be fixed later.

changeset_printer: handle workingctx in _meaningful_parentrevs()