Guard against wiki markup in commit messages

Sometimes commit messages include wiki markup [0]. Guard against
improperly interpreting this content by wrapping the commit summary in
a <nowiki> block.

* Return a structured record from useful_git_log.
* Construct output line in filter_git_output with commit subject wrapped
  in <nowiki></nowiki> tags.

[0]: https://www.mediawiki.org/w/index.php?title=MediaWiki_1.23/wmf16/Changelog&diff=prev&oldid=916766

Change-Id: I2d2ef2bbd812d7312324ff3f2e9cea85bdc57c92