Allow nested arrays in tag rendering

Summary: There's at least one of these in `phutil_tag()`, and tracking them down is a huge pain, and we've made a fair number of affordances for them elsewhere which can be simplified if we just allow them to work properly.

Test Plan: Unit tests. Diff rendering in `phutil_tag` no longer raises warnings over `hsprintf('...%s...', array())`.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4940