"%%" comment in LaTeX source breaks build.

[kfogel]

Write a comment in the LaTeX source file that starts with two percent signs ("%%") at the beginning of a line, then try to build the document with the latest (commit 801b4c3) HEAD of the jinjification branch. You'll get a long build error. In our internal OTS repository, see r15103 for details -- though note that I was able to reproduce this just now with the simple comment

%% test

...so even the full r15103 case is not necessary.