Cherry-pick elastic#10003 to 6.5: configure union merge for Changelog…

…*.next.asciidoc (elastic#10026)

Cherry-pick of PR elastic#10003 to 6.5 branch. Original message: 

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.

.gitattributes

@@ -1,5 +1,5 @@
-CHANGELOG.md  merge=union
-CHANGELOG.asciidoc  merge=union
+CHANGELOG.next.asciidoc  merge=union
+CHANGELOG-developer.next.asciidoc  merge=union
 
 # Keep these file types as CRLF (Windows).
 *.bat    text eol=crlf