The recent change to add build_refs added a new place where we include
the commit message in a zuul variable, but did not mark it unsafe,
which means ansible may try to template it. Commit messages frequently
have character sequences in them that look like jinja templates, so
for convenience, we mark them !unsafe to stop ansible from templating
them.
Correct the missing !unsafe marker, and add a test that verifies the
commit message does not appear in the vars unmarked.
Change-Id: I45ebed779884cab32718064f40716581854ea03a
d49c3a51dd