message: build: replace hard-coded master branch with the variable
  PR Close
diff: @@ -72,7 +72,7 @@ fi
  if [[ "$STATUS" != "All checks passed!" ]]; then
  echo PR $PR_NUMBER is failing with: $STATUS
  if [[ $FORCE == 1 ]]; then
  - echo FORCING: --force flag used to ignor PR status.
  + echo FORCING: --force flag used to ignore PR status.
  else
  echo Exting...
  exit 1
  @@ -100,11 +100,11 @@ CHECKOUT_MASTER="git checkout merge_pr_master"
  CHECKOUT_PATCH="git checkout merge_pr_patch"
  RESTORE_BRANCH="git checkout $CURRENT_BRANCH"
  -FETCH_PR="git fetch git@github.com:angular/angular.git pull/$PR_NUMBER/head:merge_pr heads/master:merge_pr_master heads/$PATCH_BRANCH:merge_pr_patch -f"
  +FETCH_PR="git fetch git@github.com:angular/angular.git pull/$PR_NUMBER/head:merge_pr heads/$MASTER_BRANCH:merge_pr_master heads/$PATCH_BRANCH:merge_pr_patch -f"
  BASE_PR="git checkout merge_pr~$PR_SHA_COUNT -B merge_pr_base"
  SQUASH_PR="git rebase --autosquash --interactive merge_pr_base merge_pr"
  REWRITE_MESSAGE="git filter-branch -f --msg-filter \"$BASEDIR/utils/github_closes.js $PR_NUMBER\" merge_pr_base..merge_pr"
  -PUSH_BRANCHES="git push git@github.com:angular/angular.git merge_pr_master:master merge_pr_patch:$PATCH_BRANCH"
  +PUSH_BRANCHES="git push git@github.com:angular/angular.git merge_pr_master:$MASTER_BRANCH merge_pr_patch:$PATCH_BRANCH"
  CHERRY_PICK_PR="git cherry-pick merge_pr_base..merge_pr"
  echo "======================"
  