fix: try/catch branch cache modified call

lib/workers/repository/cache.ts

@@ -53,7 +53,14 @@ async function generateBranchCache(branch: BranchConfig): Promise<BranchCache> {
       }
     }
     const automerge = !!branch.automerge;
-    const isModified = sha ? await isBranchModified(branchName) : false;
+    let isModified = false;
+    if (sha) {
+      try {
+        isModified = await isBranchModified(branchName);
+      } catch (err) /* istanbul ignore next */ {
+        // Do nothing
+      }
+    }
     const upgrades: BranchUpgradeCache[] = branch.upgrades
       ? branch.upgrades.map(generateBranchUpgradeCache)
       : [];