d/watch: Simplify watch file for gitlab.freedesktop.org

The watch file can be simplified for gitlab.freedesktop.org in the following ways:

• a filenamemangle expression is not needed, the upstream filename is good enough;
• use some special strings supported by uscan to make the file more readable.

The proposed scheme has been inspired by https://qa.debian.org/cgi-bin/watch?pkg=gitlab-shell

This also makes it easier to update the Debian package to the latest upstream version 1.1.0

Ciao, Antonio