In all=no conditionals, depend on the influences, not the test pagespec

Previously, if a page like `plugins/trail` contained a conditional like

    [[!if test="backlink(plugins/goodstuff)" all=no]]

(which it gets via `templates/gitbranch`), then the
[[plugins/conditional]] plugin would give `plugins/trail` a dependency on
`(backlink(plugins/goodstuff)) and plugins/trail`. This dependency is
useless: that pagespec can never match any page other than
`plugins/trail`, but if `plugins/trail` has been modified or deleted,
then it's going to be rendered or deleted *anyway*, so there's no point
in spending time evaluating match_backlink for it.

Conversely, the influences from the result were not taken into account,
so `plugins/trail` did not have the
`{ "plugins/goodstuff" => $DEPEND_LINKS }` dependency that it should.

Invert that, depending on the influences but not on the test.

Bug: http://ikiwiki.info/bugs/editing_gitbranch_template_is_really_slow/

IkiWiki/Plugin/conditional.pm

@@ -33,11 +33,15 @@ sub preprocess_if (@) {
 	    # An optimisation to avoid needless looping over every page
 	    # for simple uses of some of the tests.
 	    $params{test} =~ /^([\s\!()]*((enabled|sourcepage|destpage|included)\([^)]*\)|(and|or))[\s\!()]*)+$/) {
-		add_depends($params{page}, "($params{test}) and $params{page}");
 		$result=pagespec_match($params{page}, $params{test},
 				location => $params{page},
 				sourcepage => $params{page},
 				destpage => $params{destpage});
+		my $i = $result->influences;
+		foreach my $k (keys %$i) {
+			# minor optimization: influences are always simple dependencies
+			$IkiWiki::depends_simple{$params{page}}{lc $k} |= $i->{$k};
+		}
 	}
 	else {
 		$result=pagespec_match_list($params{page}, $params{test},