Issue Summary

For AUTO-MERGE pull requests, POST /merge should not be called after first attempt unless get /merge changes.

This is reproducible on Data Center: no

Steps to Reproduce

1. Create a PR with user B
2. Click on auto-merge with user A who has permission to approve PR but no permission to merge to the target branch.
3. Login via user B and view the PR again.

Expected Results

The auto-merge `POST /merge` shouldn't be called again if API is failed, unless there is change in `GET /merge `

Actual Results

The auto-merge keeps calling and failing and kind of goes into a loop.

Workaround

User B will have to go to cancel auto-merge first to allow user A to merge PR.