Going to /<package-name> should show an actual diff

[oBusk]

Currently it goes to /<package-name>@1.2.3...<package-name>@1.2.3 which has no diff since they are the same.

We should redirect to a comparison between latest-1 and latest

Related: #702

[oBusk]

This means we need special case going from versionsToSpecs into canonicalSpec. Essentially we don't want to do something "clever" if intended diff is between 1.2.3...1.2.3, only when there's only one diff to start with.

[oBusk]

The annoyng thing is that versionsToSpecs is the part that gets "both" specs, so when it gets a package name (without version) as A, and nothing as B, it should return something like package@latest~1...package@latest or something similiar. Where ~1, is a syntax where I mean "the version before latest". But since there is no such syntax in npm specs, I'm not sure how to do it.