I don't have much to say on the topic in general as I'm afraid whatever we'll do with bugs will result in a big mess anyway (I'm obviously *not* an optimist but I think of myself as being realistic ;-) ).

I wanted to comment on one specific argument, though, which might not be an advantage as "obvious" as it appears on first glance:

Am 21.10.2017 um 00:53 schrieb Marc Jeanmougin:

* The integration in both directions is non-existent : while we could,
on lp, use  stuff like "bzr commit --fixes lp:123456", and commit/bugs
would link one to the other, and while gitlab MR and bugs can also be
linked
(https://docs.gitlab.com/ce/user/project/issues/closing_issues.html#via-merge-request)
; using separate platforms prevents us to use either of those and forces
to link manually bugs urls in MR and commit URLs in bug comments.

While such integrations are nice in principle, they are actually the source of a lot of problems:

For example you cited Bazaar's --fixes switch. It was great while we were on bzr, but when we moved to GitLab we almost lost this information! Only by hacking the export routine we were able to save this data and migrate it.
Even with that information secured there are still a lot of commit messages simply stating "fixes bug 123456" which leaves one searching for the actual launchpad bug manually.
Luckily for launchpad a manual search is actually possible as bug numbers are global and do not depend on the project. For GitLab issues are counted per-project (and we already have several) so it might quickly become a mess if we rely on automatic linking. (Just imagine what happens if your local git client tells you issue #123 was fixed - it will be fun to find out which project this refers to and derive a bug link from that)
An this is just now! Imagine the chaos that will result if we switch our VCS again at some point (hopefully far into) the future.

With this in mind the apparent advantage from automatic linking integrations is diminished significantly (if not even negated) and for these reasons I actually want to *encourage* everybody to actively use full URLs where reasonable (especially in commit messages).

While it might be a tiny bit more work initially (most of the time the full link is available for copy/pasting though, so it really doesn't matter), it actually saves a lot of time when trying to work with that information afterwards, especially if *not* working on GitLab. This way we actually become independent from such platforms GitLab and produce helpful commit data, not only for us but also for the generations after us!