How to trigger a build where its dependencies use a different revision

It looks like this could possibly be done by first triggering the upstream build on some commit/branch and then using a build promotion to trigger the downstream build on a different commit/branch, but I don't think there is any build promotion REST API?

I've also tried triggering the build like this via curl, but it seems to ignore the snapshot dependencies. A build IS triggered, but not with this snapshot dependency, it uses one from the branch name.

```

curl -X POST -u "username:password" http://localhost:8111/app/rest/buildQueue --header "Content-Type: application/xml" -H "Accept: application/json" \
-d "
<build branchName=\"TRIGGERED_BUILD_BRANCH_NAME\">
  <buildType id=\"TRIGGERED_BUILD_TYPE_ID\" />
  <snapshot-dependencies>
    <build locator=\"id:SNAPSHOT_BUILD_ID\">
      <buildType id=\"SNAPSHOT_BUILD_TYPE_ID\" />
    </build>
  </snapshot-dependencies>
</build>
"

```

Perhaps by using a different vcsRoot for both builds (even though they use the same repository) I can somehow make the upstream build use a different commit/branch? Although I don't want it to use the default branch in the upstream dependency - I'd like to be able to specify the branch/commit for both the triggered build and a different one for it's upstream dependency.