test_git_push: Adds a test for branch creation

This tests `git push` attempting to create a branch when the branch
already unexpectedly exists on the remote. This should (and does)
fail.

cli/tests/test_git_push.rs

@@ -349,6 +349,41 @@ fn test_git_push_deletion_unexpectedly_moved() {
     "###);
 }
 
+#[test]
+fn test_git_push_creation_unexpectedly_already_exists() {
+    let (test_env, workspace_root) = set_up();
+
+    // Forget branch1 locally
+    test_env.jj_cmd_ok(&workspace_root, &["branch", "forget", "branch1"]);
+
+    // Create a new branh1
+    test_env.jj_cmd_ok(&workspace_root, &["new", "root()", "-m=new branch1"]);
+    std::fs::write(workspace_root.join("local"), "local").unwrap();
+    test_env.jj_cmd_ok(&workspace_root, &["branch", "create", "branch1"]);
+    let assert = test_env
+        .jj_cmd(&workspace_root, &["branch", "list", "--all"])
+        .assert()
+        .success();
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    branch1: yostqsxw 4c595cf9 new branch1
+    branch2: rlzusymt 8476341e (empty) description 2
+      @origin: rlzusymt 8476341e (empty) description 2
+    "###);
+    insta::assert_snapshot!(get_stderr_string(&assert), @"");
+
+    let assert = test_env
+        .jj_cmd(&workspace_root, &["git", "push"])
+        .assert()
+        .failure();
+    insta::assert_snapshot!(get_stdout_string(&assert), @"");
+    insta::assert_snapshot!(get_stderr_string(&assert), @r###"
+    Branch changes to push to origin:
+      Add branch branch1 to 4c595cf9ac0a
+    Error: The push conflicts with changes made on the remote (it is not fast-forwardable).
+    Hint: Try fetching from the remote, then make the branch point to where you want it to be, and push again.
+    "###);
+}
+
 #[test]
 fn test_git_push_locally_created_and_rewritten() {
     let (test_env, workspace_root) = set_up();