fix(release): use git push --atomic

That way it won't leave the repo in an inconsistent state.

It is supported by GitHub[1].

[1]: https://github.blog/2015-04-30-git-2-4-atomic-pushes-push-to-deploy-and-more/

(cherry picked from commit 40493de)

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

tools/release-tag.sh

@@ -61,5 +61,5 @@ warn "!WARNING! The next step will modify upstream: $UPSTREAM_URL by running:"
 echo "    git push $UPSTREAM v$version"
 echo "    git push $UPSTREAM $RELEASE_BRANCH"
 get_user_confirmation || die "Cancelling tag push"
-git push $UPSTREAM "v$version"
-git push $UPSTREAM "$RELEASE_BRANCH"
+git push --atomic $UPSTREAM "v$version"
+git push --atomic $UPSTREAM "$RELEASE_BRANCH"