snapshot uploading: fix the ahead/behind by logic

The logic was _exactly_ inverted. What it _did_ want to verify was that
the commit for which the snapshot was built is reachable from `main`.
What it verified instead was that `main`'s tip commit is reachable from
said commit. 🤦

I noticed this only today, when a successful `git-artifacts` run in the
v2.48.1 PR at git-for-windows/git#5411 tried to
upload a snapshot, and the (correct) ahead/behind by logic in the
`upload-artifacts` workflow failed (but then succeeded when I re-ran the
workflow after releasing Git for Windows v2.48.1).

Let's invert the logic so that it does what it is supposed to do.

Signed-off-by: Johannes Schindelin <[email protected]>

GitForWindowsHelper/cascading-runs.js

@@ -166,13 +166,14 @@ const cascadingRuns = async (context, req) => {
 
             // Next, check that the commit is on the `main` branch
             const gitToken = await getToken(context, checkRunOwner, checkRunRepo)
-            const { behind_by } = await githubApiRequest(
+            const { ahead_by } = await githubApiRequest(
                 context,
                 gitToken,
                 'GET',
                 `/repos/${checkRunOwner}/${checkRunRepo}/compare/HEAD...${commit}`,
+                // `/repos/dscho/git/compare/HEAD...${commit}`,
             )
-            if (behind_by > 0) {
+            if (ahead_by > 0) {
                 return `Ignoring ${name} check-run because its corresponding commit ${commit} is not on the main branch`
             }
 

__tests__/index.test.js

@@ -175,7 +175,7 @@ The \`git-artifacts-aarch64\` workflow run [was started](dispatched-workflow-git
     }
     if (method === 'GET' && requestPath ===
         '/repos/git-for-windows/git/compare/HEAD...0c796d3013a57e8cc894c152f0200107226e5dd1') {
-        return { behind_by: 0 }
+        return { ahead_by: 0 }
     }
     throw new Error(`Unhandled ${method}-${requestPath}-${JSON.stringify(payload)}`)
 })