git: skip branches on root commit on export

Git doesn't have a root commit, so we should skip branches pointing to
it on export, just like we do with conflicted branches (which Git also
doesn't support).

lib/src/git.rs

@@ -395,6 +395,8 @@ pub enum FailedRefExportReason {
     /// The ref was in a conflicted state from the last import. A re-import
     /// should fix it.
     ConflictedOldState,
+    /// The branch points to the root commit, which Git doesn't have
+    OnRootCommit,
     /// We wanted to delete it, but it had been modified in Git.
     DeletedInJjModifiedInGit,
     /// We wanted to add it, but Git had added it with a different target
@@ -435,6 +437,7 @@ pub fn export_some_refs(
     let mut branches_to_update = BTreeMap::new();
     let mut branches_to_delete = BTreeMap::new();
     let mut failed_branches = vec![];
+    let root_commit_target = RefTarget::normal(mut_repo.store().root_commit_id().clone());
     let view = mut_repo.view();
     let jj_repo_iter_all_branches = view.branches().iter().flat_map(|(branch, target)| {
         itertools::chain(
@@ -483,6 +486,14 @@ pub fn export_some_refs(
         if new_branch == old_branch {
             continue;
         }
+        if *new_branch == root_commit_target {
+            // Git doesn't have a root commit
+            failed_branches.push(FailedRefExport {
+                name: jj_known_ref,
+                reason: FailedRefExportReason::OnRootCommit,
+            });
+            continue;
+        }
         let old_oid = if let Some(id) = old_branch.as_normal() {
             Some(Oid::from_bytes(id.as_bytes()).unwrap())
         } else if old_branch.has_conflict() {

lib/tests/test_git.rs

@@ -27,8 +27,8 @@ use jj_lib::commit::Commit;
 use jj_lib::commit_builder::CommitBuilder;
 use jj_lib::git;
 use jj_lib::git::{
-    FailedRefExportReason, GitFetchError, GitImportError, GitPushError, GitRefUpdate,
-    SubmoduleConfig,
+    FailedRefExport, FailedRefExportReason, GitFetchError, GitImportError, GitPushError,
+    GitRefUpdate, SubmoduleConfig,
 };
 use jj_lib::git_backend::GitBackend;
 use jj_lib::op_store::{BranchTarget, RefTarget};
@@ -1332,6 +1332,28 @@ fn test_export_conflicts() {
     );
 }
 
+#[test]
+fn test_export_branch_on_root_commit() {
+    // We skip export of branches pointing to the root commit
+    let test_data = GitRepoData::create();
+    let git_repo = test_data.git_repo;
+    let mut tx = test_data
+        .repo
+        .start_transaction(&test_data.settings, "test");
+    let mut_repo = tx.mut_repo();
+    mut_repo.set_local_branch_target(
+        "on_root",
+        RefTarget::normal(mut_repo.store().root_commit_id().clone()),
+    );
+    assert_eq!(
+        git::export_refs(mut_repo, &git_repo),
+        Ok(vec![FailedRefExport {
+            name: RefName::LocalBranch("on_root".to_string()),
+            reason: FailedRefExportReason::OnRootCommit,
+        }])
+    );
+}
+
 #[test]
 fn test_export_partial_failure() {
     // Check that we skip branches that fail to export