Rebuild pages that change their type.

Previously, if a page changed its type but not its mtime
(e.g. mv page.txt page.mdwn), then it would not be rebuilt.

Now, check if the source of a page has changed,
in which case force a rebuild of that page.
(cherry picked from commit b6a3b8a683fed7a7f6d77a5b3f2dfbd14c849843)

IkiWiki/Render.pm

@@ -319,6 +319,10 @@ sub refresh () { #{{{
 	# check for added or removed pages
 	foreach my $file (@files) {
 		my $page=pagename($file);
+		if (exists $pagesources{$page} && $pagesources{$page} ne $file) {
+			# the page has changed its type
+			$forcerebuild{$page}=1;
+		}
 		$pagesources{$page}=$file;
 		if (! $pagemtime{$page}) {
 			if (isinternal($page)) {