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.