hgweb: drop unused argument 'tmpl' from webutil.diffstat()

hgweb: use template context to render {diffstat} This is a preferred way to process nested templates.

hgweb: wrap {diffstat} with mappedgenerator This can't be a mappinggenerator since the associated templates are switched per item. We already have webutil.diffstatgen(). That's why the generator function is named as _diffstattmplgen().

pathencode: improve error messages slightly Both of these are conditions we should never hit, so we can afford to be more verbose to make debugging less baffling. Differential Revision: https://phab.mercurial-scm.org/D3589