git: on import, only add ref target as head if target changed (#44)

If you import Git refs, then rebase a commit pointed to by some Git
ref, and then re-import Git refs, you don't want the old commit to be
made a visible head again. That's particularly annoying when Git refs
are automatically updated by every command.

lib/src/git.rs

@@ -83,14 +83,14 @@ pub fn import_refs(
             }
         };
         let id = CommitId::from_bytes(git_commit.id().as_bytes());
-        let commit = store.get_commit(&id).unwrap();
-        mut_repo.add_head(&commit);
         // TODO: Make it configurable which remotes are publishing and update public
         // heads here.
         mut_repo.set_git_ref(full_name.clone(), RefTarget::Normal(id.clone()));
         let old_target = existing_git_refs.remove(&full_name);
-        let new_target = Some(RefTarget::Normal(id));
+        let new_target = Some(RefTarget::Normal(id.clone()));
         if new_target != old_target {
+            let commit = store.get_commit(&id).unwrap();
+            mut_repo.add_head(&commit);
             changed_git_refs.insert(full_name, (old_target, new_target));
         }
     }

lib/tests/test_git.rs

@@ -236,6 +236,27 @@ fn test_import_refs_reimport() {
     );
 }
 
+#[test]
+fn test_import_refs_reimport_head_removed() {
+    // Test that re-importing refs doesn't cause a deleted head to come back
+    let settings = testutils::user_settings();
+    let test_workspace = testutils::init_repo(&settings, true);
+    let repo = &test_workspace.repo;
+    let git_repo = repo.store().git_repo().unwrap();
+
+    let commit = empty_git_commit(&git_repo, "refs/heads/main", &[]);
+    let mut tx = repo.start_transaction("test");
+    git::import_refs(tx.mut_repo(), &git_repo).unwrap();
+    let commit_id = CommitId::from_bytes(commit.id().as_bytes());
+    // Test the setup
+    assert!(tx.mut_repo().view().heads().contains(&commit_id));
+
+    // Remove the head and re-import
+    tx.mut_repo().remove_head(&commit_id);
+    git::import_refs(tx.mut_repo(), &git_repo).unwrap();
+    assert!(!tx.mut_repo().view().heads().contains(&commit_id));
+}
+
 fn git_ref(git_repo: &git2::Repository, name: &str, target: Oid) {
     git_repo.reference(name, target, true, "").unwrap();
 }