changelog conflicts are a thing of the past .. [EDIT: in local git checkouts, not in github]

[timotheecour]

/cc @Araq

• I've tried on a dummy repo, this works well and prevents virtually all future changelog conflicts. The only caveat is mentioned in PR content, and is a very good tradeoff.
• this PR is orthogonal to avoid messing up changelog history on each subsequent release #14025 (only thing is that .gitattributes file will need to be updated after avoid messing up changelog history on each subsequent release #14025 is merged)

CI failures unrelated

alternatives considered

• gitatributes merge=union (this PR)
• gitlab's placeholder approach (https://about.gitlab.com/blog/2015/02/10/gitlab-reduced-merge-conflicts-by-90-percent-with-changelog-placeholders/)
• 1 file-per-changelog-entry (https://medium.com/@nettsundere/on-reducing-changelog-merge-conflicts-1eb23552630b, and IIRC git repo itself; see also http://seanwallis.com/post/resolving-changelog-merge-conflict-madness/)
• custom gitattributes merge-changelog Add git-merge-changelog support (custom merge driver in .gitattributes) isaacs/github#560 (not a builtin gitattributes)

the 1 file-per-changelog-entry can be tried if merge=union turns out bad

see also https://github.com/git/git/tree/master/Documentation/RelNotes (not quite same as 1-per-entry but git issues more frequent releases, ie patch releases, eg:

Documentation/RelNotes/2.25.0.txt
Documentation/RelNotes/2.25.1.txt
Documentation/RelNotes/2.25.2.txt
Documentation/RelNotes/2.25.3.txt

[EDIT] works locally, but not on github UI

this avoids merge conflicts in your local git client but not in github UI, as also noted in isaacs/github#487 (unlike gitlab which supports it
but whoever's merging the PR should be able to easily merge through "resolve conflict" in those cases; if not, a local rebase (which should give no conflict) + git push -f will fix it

[Araq]

Irrelevant CI failures, let's give it a try.

[timotheecour]

first casuality: my own PR that i just rebased, causing undesired "silent" merge, see #13265 (comment) for context + recommendation. The idea is sound, but it needs to have changelog.md moved to changelogs/changelog_1_4.md (the earlier the better to avoid similar issues)

[timotheecour]

see added note above (works locally, but not on github UI); at least, conflicts only happen on merge, not on local client when they rebase to devel.
(see note for how to resolve UI conflict)