make-snapshot: generate ChangeLog iff not exist
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59291 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
make-snapshot: generate ChangeLog iff not exist
revisions, but not doc/ChangeLog-YYYY. fix for 2..3.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59291 b2dd03c8-39d4-4d8f-98ff-823fe69b080e