Markdown.pl: correct comment sanitation

The XML standard section 2.5 is quite specific:

  the string "--" (double-hyphen) MUST NOT occur within comments

In fact, xmllint will complain about any comments that
incorrectly contain an internal "--" sequence as they are
not valid XML.

Adjust the sanitation code to only pass through valid XML
comments using the same pattern that _HashHTMLBlocks uses
to recognize them.

With this change, invalid XML comments will be treated as
literal text by the sanitizer and have the initial "<" escaped
to &lt; thus rendering them as not a comment at all.

Also take this opportunity to correct the comments in the
_HashHTMLBlocks function from "HTML" to "XML" to reflect
what it actually matches.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -834,7 +834,7 @@ sub _HashHTMLBlocks {
 		"\n\n" . $key . "\n\n";
 	    }eigx;
 
-    # Special case for standalone HTML comments:
+    # Special case for standalone XML comments:
     $text =~ s{
 		(?:
 		    (?<=\n\n)	    # Starting after a blank line
@@ -2535,12 +2535,13 @@ sub _SanitizeTags {
 	    next;
 	}
 	my $tstart = pos($text);
-	if ($text =~ /\G(<[^>]*>)/gc) {
-	    my $tag = $1;
-	    if ($tag =~ /^<!--/) { # pass "comments" through
-		$ans .= $tag;
+	if ($text =~ /\G(<!--(?:[^-]|(?:-(?!-)))*-->)/gc) {
+	    # pass "comments" through
+	    $ans .= $1;
 	    next;
 	}
+	if ($text =~ /\G(<[^>]*>)/gc) {
+	    my $tag = $1;
 	    my $tt;
 	    if (($tag =~ m{^<($g_possible_tag_name)(?:[\s>]|/>$)} ||
 		 $tag =~ m{^</($g_possible_tag_name)\s*>}) &&