Respect GitLab squash settings without squashing ucascade cascade merges

[4shurik]

When the GitLab project setting Squash commits when merging is set to Encourage, user merge requests into release branches should still be squashed.

However, ucascade cascade merges between release branches and into master should remain regular merge commits. If cascade merges are squashed, the same changes can appear twice in the git graph through unrelated histories: once as a squash commit on the target branch and later again through the original merge
commit propagated by subsequent release merges.

This change makes squash behavior configurable so cascade merges can preserve graph continuity while regular user merge requests can still follow the project squash policy.

[4shurik]

Hi @jmini @fbpe @gysel 👋

Gentle nudge on this one — it's been open since April, so I just wanted to make sure it hasn't slipped off the radar.

Quick recap: the change makes ucascade respect GitLab's "Squash commits when merging" setting for regular MRs, while keeping the automated cascade merges between release branches (and into master) as plain merge commits — so the history doesn't end up with duplicated changes.

It's a single, self-contained commit, and I'm happy to rebase or address any feedback whenever it's convenient for you.