Fix comment collapsing, and use spaces before void tag trailing slash

The PHP parser strips lines with a single comment and ws followed by a newline
(see Preprocessor_DOM.php around line 36).

A space before the void trailing slash seems to be slightly more common in
wikitext, which avoids some syntactic diffs.

Overall two more wt2wt and two more wt2html tests are passing.

Change-Id: I0dc4cd577c9249f22c5e79cd9b69046ad6d919cf
Merge "Fix comment collapsing, and use spaces before void tag trailing slash"