message: set label via bot
diff: @@ -28,15 +28,19 @@ queue has been merged.
  1. Using the interface on [Homu queue], select a few pull requests and then use
  "rollup" button to make one. (The text about fairness can be ignored.)
  -2. Use the `@bors r+ rollup=never p=<NUMBER_OF_PRS_IN_ROLLUP>` command in the
  - pull request thread.
  -3. Mark the pull request with the label `rollup`.
  -4. If the rollup fails, use the logs rust-highfive (really it is
  +2. Run the following command in the pull request thread:
  +
  + ```
  + @rustbot modify labels: +rollup
  + @bors r+ rollup=never p=<NUMBER_OF_PRS_IN_ROLLUP>
  + ````
  +
  +3. If the rollup fails, use the logs rust-highfive (really it is
  rust-log-analyzer) provides to bisect the failure to a specific PR and do
  `@bors r-`. If the PR is running, you need to do `@bors r- retry`. Otherwise,
  your rollup succeeded. If it did, proceed to the next rollup (every now and
  then let `rollup=never` and toolstate PRs progress).
  -5. Recreate the rollup without the offending PR starting again from **1.**
  +4. Recreate the rollup without the offending PR starting again from **1.**
  ## Selecting Pull Requests
  