"Read more" shortening for trait impls chops off link references

[clarfonthey]

Here's an example: https://doc.rust-lang.org/nightly/std/option/enum.Option.html#impl-Hash

If you notice here, because rustdoc naïvely chops off all lines of the doc but the first, link references get lost. While this particular instance will be solved by autolinking, this doesn't work if you link to something in general.

[clarfonthey]

Managed to isolate this to fn shorter in librustdoc/html/render.rs. Going to try a PR soon.

…or not. I'm going to have to take a bit more of a look to understand the stuff surrounding plain_summary_line. It looks like a lot of it could use some refactoring.

[euclio]

This is also present in the Windows CommandExt impl for std::process::Command: https://doc.rust-lang.org/std/process/struct.Command.html#implementations