Use union merge for spec/truffleruby.next-specs

Author: eregon

.gitattributes

@@ -2,6 +2,8 @@
 
 /CHANGELOG.md merge=union
 
+/spec/truffleruby.next-specs merge=union
+
 # Rules for GitHub's Linguist language-classification system. We're abusing the
 # 'vendored' attribute to exclude files as a lot of this isn't really vendored,
 # and a whole lot of actually vendored code isn't listed! What we want to do is