Detect the branch from Vercel env var (set during build)

Author: matt-aitken

packages/core/src/v3/apiClient/getBranch.ts

@@ -12,12 +12,18 @@ export function getBranch({
     return specified;
   }
 
-  // not specified, so detect from process.env
+  // not specified, so detect our variable from process.env
   const envVar = getEnvVar("TRIGGER_PREVIEW_BRANCH");
   if (envVar) {
     return envVar;
   }
 
+  // detect the Vercel preview branch
+  const vercelPreviewBranch = getEnvVar("VERCEL_GIT_COMMIT_REF");
+  if (vercelPreviewBranch) {
+    return vercelPreviewBranch;
+  }
+
   // not specified, so detect from git metadata
   if (gitMeta?.commitRef) {
     return gitMeta.commitRef;