git: fix regression in pruning of remote refs

A while ago, I replaced a call to git2-rs's `Remote::fetch()` by calls
to `Remote::download()` and `Remote::update_tips()`. The function is
documented to be a convenience for those function, but it turns out
that the pruning of deleted remote refs is a separate call
(`Remote::prune()`), so we need to call that too.

lib/src/git.rs

@@ -14,7 +14,7 @@
 
 use std::collections::{BTreeMap, HashSet};
 
-use git2::{FetchPrune, RemoteCallbacks};
+use git2::RemoteCallbacks;
 use itertools::Itertools;
 use thiserror::Error;
 
@@ -143,10 +143,10 @@ pub fn fetch(
     fetch_options.proxy_options(proxy_options);
     let callbacks = create_remote_callbacks();
     fetch_options.remote_callbacks(callbacks);
-    fetch_options.prune(FetchPrune::On);
     let refspec: &[&str] = &[];
     remote.download(refspec, Some(&mut fetch_options))?;
     remote.update_tips(None, false, git2::AutotagOption::Unspecified, None)?;
+    remote.prune(None)?;
     // TODO: We could make it optional to get the default branch since we only care
     // about it on clone.
     let mut default_branch = None;

lib/tests/test_git.rs

@@ -327,6 +327,37 @@ fn test_fetch_success() {
     );
 }
 
+#[test]
+fn test_fetch_prune_deleted_ref() {
+    let settings = testutils::user_settings();
+    let temp_dir = tempfile::tempdir().unwrap();
+    let source_repo_dir = temp_dir.path().join("source");
+    let clone_repo_dir = temp_dir.path().join("clone");
+    let jj_repo_dir = temp_dir.path().join("jj");
+    let source_git_repo = git2::Repository::init_bare(&source_repo_dir).unwrap();
+    empty_git_commit(&source_git_repo, "refs/heads/main", &[]);
+    let clone_git_repo =
+        git2::Repository::clone(source_repo_dir.to_str().unwrap(), &clone_repo_dir).unwrap();
+    std::fs::create_dir(&jj_repo_dir).unwrap();
+    let jj_repo = ReadonlyRepo::init_external_git(&settings, jj_repo_dir, clone_repo_dir).unwrap();
+
+    let mut tx = jj_repo.start_transaction("test");
+    git::fetch(tx.mut_repo(), &clone_git_repo, "origin").unwrap();
+    // Test the setup
+    assert!(tx.mut_repo().get_branch("main").is_some());
+
+    // After re-fetching, the branch should be deleted
+    source_git_repo
+        .find_reference("refs/heads/main")
+        .unwrap()
+        .delete()
+        .unwrap();
+    git::fetch(tx.mut_repo(), &clone_git_repo, "origin").unwrap();
+    assert!(tx.mut_repo().get_branch("main").is_none());
+
+    tx.discard();
+}
+
 #[test]
 fn test_fetch_no_default_branch() {
     let settings = testutils::user_settings();