textproc/mgdiff: move source to public VCS; Codeberg

The original source location went dark. This patch moves the
original source to a public VCS, as it's still a suprisingly
popular utility. This patch changes MASTERSITES, WWW and distinfo.
While here, use desktop icon now located with the source.

PR: 280289

(cherry picked from commit ebd88827296f5c4f1b68c87f882886ffb199b5cd)