fix(git): add branchExists safety check to isBranchModified

lib/util/git/index.spec.ts

@@ -136,6 +136,9 @@ describe('platform/git', () => {
     });
   });
   describe('isBranchModified()', () => {
+    it('should return false when branch is not found', async () => {
+      expect(await git.isBranchModified('renovate/not_found')).toBe(false);
+    });
     it('should return true when author matches', async () => {
       expect(await git.isBranchModified('renovate/future_branch')).toBe(false);
       expect(await git.isBranchModified('renovate/future_branch')).toBe(false);

lib/util/git/index.ts

@@ -401,6 +401,13 @@ export async function isBranchModified(branchName: string): Promise<boolean> {
   if (config.branchIsModified[branchName] !== undefined) {
     return config.branchIsModified[branchName];
   }
+  if (!branchExists(branchName)) {
+    logger.debug(
+      { branchName },
+      'Branch does not exist - cannot check isModified'
+    );
+    return false;
+  }
   // Retrieve the author of the most recent commit
   const lastAuthor = (
     await git.raw([