obslog: make content and description patches available to templater The code was repeatedly calling fm.write() with the same field ("patch" and "descdiff"). I think that led to the value constantly getting replaced, so when it was used in a template (as {patch} or {descdiff}), it would only get the last value, which was always an empty string. This patch fixes it by writing the full patch to a temporary buffer and then assigning the whole patch to the formatter field.

obslog: rename {desc-diff} to {descdiff} since the former is invalid

tests: demonstrate broken {patch} and {desc-diff} template keywords