Middle-clicking on search results should not load the link on the same page

Middle clicking is usually used for opening a link in a new tab in most browsers. It's an easy way to navigate without losing your current page.

When using rustdoc's search, I'd like to be able to middle-click on search results so that I can open multiple results in new tabs. However, currently the `onclick` listener captures middle clicks too, so link opens in two places — in a new tab, and on the same page.

This partially fixes it. It would be better if the anchor tag on a result spanned the entire result path+name, and not just the name -- while normal clicking still works due to the event handler, middle clicking is broken if you click on something other than the result name (not the path). I don't mind doing it that way, but it seems like there was a reason behind having the `a` tag only cover the name. ([Code pointer](https://github.com/rust-lang/rust/blob/be6c7cf882bacc9da96df5304679af678390d5b0/src/librustdoc/html/static/main.js#L464) in case I need it in the future)