Add dry run mode to npm version (#2436)

* Add DRY_RUN flag to skip publish/push

* Allow overriding branch names for testing

scripts/postversion.sh

@@ -1,17 +1,22 @@
 #!/bin/bash
 set -e
 PACKAGE_VERSION=$(node -p -e "require('./package.json').version")
-ARCHIVE_BRANCH="releases"
+ARCHIVE_BRANCH=${ARCHIVE_BRANCH:-releases}
 SOURCE_BRANCH=$(git rev-parse --abbrev-ref HEAD)
 
 echo 'postversion tasks'
 
 # npm publish will generate the pkg/sinon.js that we use below
 echo 'publish to npm'
-git push --follow-tags
-npm publish
+if [ -n "$DRY_RUN" ]; then
+    npm publish --dry-run
+else
+    git push --follow-tags
+    npm publish
+fi
 
 # Now update the releases branch and archive the new release
+echo "archiving release from $SOURCE_BRANCH to $ARCHIVE_BRANCH"
 git checkout $ARCHIVE_BRANCH
 git merge --no-edit -m "Merge version $PACKAGE_VERSION" $SOURCE_BRANCH
 
@@ -27,5 +32,5 @@ cp "pkg/sinon.js" "./docs/releases/sinon-$PACKAGE_VERSION.js"
 git add "docs/releases/sinon-$PACKAGE_VERSION.js"
 git commit -n -m "Add version $PACKAGE_VERSION to releases"
 
-git push
+[ -n "$DRY_RUN" ] || git push
 git checkout $SOURCE_BRANCH