Improve copy/paste from Diviner
Summary: Change <code> tag to <pre> and modify the related test cases
Test Plan: Run ../arcanist/bin/arc unit /home/hwang/devtools/libphutil/src/markup/engine/remarkup/blockrule/remarkupcode
Reviewers: epriestley
Reviewed By: epriestley
CC: jungejason, aran
Maniphest Tasks: T207
Differential Revision: https://secure.phabricator.com/D2067