Bug#1120328: gdm3: Use upstream's xsession file

[Jeremy Bícha]

Source: gdm3
Version: 49.1-6
Tags: forky sid patch
Forwarded: https://salsa.debian.org/gnome-team/gdm/-/merge_requests/36
X-Debbugs-CC: alessandro.astone at canonical.com

Debian has been shipping a custom Xsession file (in the packaging as
debian/Xsession) since at least 2007. Upstream gdm has provided its
own Xsession file since it was initially created. Ubuntu has been
using upstream's Xsession file since at least 2011.

I think it would be beneficial to use the cross-distro Xsession file
so that we can benefit from improvements there.

It's complicated to diff debian/Xsession with data/Xsession.in and
understand the differences. Looking at the git log of the 2 files
helps some, but the divergence was a LONG time ago.

I did recognize that we probably want
https://salsa.debian.org/gnome-team/gdm/-/commit/1cdc3f05c2

Ubuntu has some patches that I'm copying over.

I'm submitting a Salsa merge request:
https://salsa.debian.org/gnome-team/gdm/-/merge_requests/36

Thanks,
Jeremy Bícha