pin: Fix the template.

In APT preferences files the only allowed comments are lines that start
with `Explanation:`, commented lines that start with a # trigger a
myriad of interesting bugs. This is considered a feature of APT.

Because we're only ever writing a single file at a time with only a #
comment at the top we were getting away with this but it shouldn't be
there in the first place.
pin: caller_module_name is not a topscope variable
Merge pull request #280 from daenney/fix-pin-comments

Fix pin comments