rustdoc: back navigation in jump to definition is sometimes broken

[jsha]

Steps to reproduce (Chrome / Linux):

1. Visit https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/index.html
2. Click source link (https://doc.rust-lang.org/nightly/nightly-rustc/src/rustdoc/clean/mod.rs.html#1-2243)
3. Scroll down in page until you see a link that goes elsewhere within the same file (e.g. doc on line 104)
4. Click the link.
5. Navigate back in the browser.

Expected result:

The browser now shows the scroll position from before clicking the link.

Actual result:

The scroll position is the top of the page.

This seems to only happen when the first navigation included a fragment (e.g. #1-2243 in this example)

/cc @GuillaumeGomez