fix: add missing await to getBranchPr call (#986)

Closes #983

lib/workers/branch/automerge.js

@@ -8,7 +8,7 @@ async function tryBranchAutomerge(config) {
   if (!config.automerge || config.automergeType === 'pr') {
     return 'no automerge';
   }
-  const existingPr = config.api.getBranchPr(config.branchName);
+  const existingPr = await config.api.getBranchPr(config.branchName);
   if (existingPr) {
     return 'automerge aborted - PR exists';
   }