Generated Launchpad URL to changelog is incorrect when a binary package version isn't the same as its source package version

[Robie Basak]

As reported by Luke at https://lists.ubuntu.com/archives/ubuntu-release/2019-March/004738.html:

update-manager assumes that it can generate a Launchpad +changelog URL from a binary package version, instead of mapping from the corresponding source package version. In cases where these aren't the same, the URL 404s.

Luke's example is:

> "Changes for grub-efi-amd64-signed versions:
> Installed version: 1.66.20+2.02~beta2-36ubuntu3.20
> Available version: 1.66.21+2.02~beta2-36ubuntu3.21
>
> The list of changes is not yet available.
>
> Please use http://launchpad.net/ubuntu/+source/grub2-signed/1.66.21+2.02~beta2-36ubuntu3.21/+changelog
> until the changes become available or try again later."

[Robie Basak (racb)]

This presumably affects Xenial since 1.66.21+2.02~beta2-36ubuntu3.21 is published in xenial-proposed only currently. The latest version of update-manager on Xenial at the moment is 1:16.04.15 from xenial-updates.