* tool/merger.rb: add interaction when only ChangeLog is modified.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39358 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Add test for [Backport #7896] [ruby-core:52607]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39359 b2dd03c8-39d4-4d8f-98ff-823fe69b080e