configure union merge for Changelog*.next.asciidoc

[urso]

This change automatically merges Changelog*.next.asciidoc files when
merging or rebasing via merge=union. No more need to fix changelog
files when doing git rebase master.

Note: github UI seems to ignore the setting. Convenience will only be improved on command line.

[ruflin]

This is the entry that always creates the "interesting" rebases in the changelog. So I wonder if we rather leave it out instead of giving devs the impressions a clean rebase happened.

[ruflin]

@urso I assume the label should be needs_backport not backport

[urso]

This is the entry that always creates the "interesting" rebases in the changelog

True. But with the move to Changelog.next.asciidoc the wrongly rebase is local to the current release only. We still checks for missing entries and wrong entries if really required. So far this is hasn't been an issue if entries did stay in the correct release after rebase.

Even without union merge the line can move to an unexpected location. As developer you will look for <<<<< and then just remove <<<<<<, ===========, and >>>>>>>>, such that both entries appear in the merged changelog. Rarely does one check the if the conflict is really at the right location.

I'm more tempted to remove union-merge from the Changelog.asciidoc file itself.

[ph]

I'm more tempted to remove union-merge from the Changelog.asciidoc file itself.

I would be OK with that, I've made a few mistakes in changelog entries, now to be honest I always diff the change locally before creating the backports.

[urso]

@ph @kvch I removed union merge from Changelog.asciidoc, so to force devs to have a look when Changelog.asciidoc gets merged. Please have another look.