x/pkgsite: broken link to unknown branch

[katiehockman]

What is the URL of the page with the issue?

https://pkg.go.dev/cmd/vet/whitelist

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36

What did you do?

Clicked the "vet" link at the top right of https://pkg.go.dev/cmd/vet/whitelist

What did you expect to see?

I would expect the link to take me to https://pkg.go.dev/cmd/vet (which resolves) instead of https://pkg.go.dev/cmd/vet@unknown, which is a 404.

What did you see instead?

A 404 at https://pkg.go.dev/cmd/vet@unknown.

@julieqiu @jba

[gopherbot]

Change https://golang.org/cl/260437 mentions this issue: internal/frontend: fix breadcrumbs for stdlib