README that is not the first chapter confuses the current page highlight

[ehuss]

Problem

If a chapter named README.md is not the first chapter, the sidebar gets confused as to which page is currently active.

Steps

With a SUMAMRY.md outline such as:

# Summary

[Prefix 1](prefix-1.md)
[Prefix 2](prefix-2.md)

- [Introduction](README.md)

when loading index.html, both "Prefix 1" and "Introduction" are highlighted as the current page.

Only "Introduction" should be highlighted.

Possible Solution(s)

No response

Notes

No response

Version

mdbook v0.5.0-alpha.1