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/