[ci skip] port gen-changelog from 1.x

util/gen-changelog.sh

@@ -6,9 +6,6 @@ end="$2"
 
 enddate=`git log -1 ${end} --pretty="%ai" | cut -d ' ' -f 1`
 
-startrev="`git show ${start}:build.log | awk '/polymer: / {print $2}'`"
-endrev="`git show ${end}:build.log | awk '/polymer: / {print $2}'`"
-
 old=""
 if [ -e CHANGELOG.md ]; then
   old="`sed -e '1,2d' CHANGELOG.md`"
@@ -18,7 +15,7 @@ cat > CHANGELOG.md <<EOD
 # Change Log
 
 ## [${end}](https://github.com/Polymer/polymer/tree/${end}) (${enddate})
-`git log --no-merges "${startrev}".."${endrev}^1" --pretty="${PRETTY}"`
+`git log --no-merges "${start}".."${end}^1" --pretty="${PRETTY}"`
 
 ${old}
 EOD