cli: correctly update to remote's default branch after clone

It turns out that `FETCH_HEAD` is not the remote's `HEAD` (it's
actually not even a normal symbolic ref; it contains many lines of
commits and names). We're supposed to ask the remote for its default
branch instead. That's what this patch does.

lib/src/git.rs

@@ -127,7 +127,7 @@ pub fn fetch(
     mut_repo: &mut MutableRepo,
     git_repo: &git2::Repository,
     remote_name: &str,
-) -> Result<(), GitFetchError> {
+) -> Result<Option<String>, GitFetchError> {
     let mut remote =
         git_repo
             .find_remote(remote_name)
@@ -149,10 +149,22 @@ pub fn fetch(
     fetch_options.prune(FetchPrune::On);
     let refspec: &[&str] = &[];
     remote.fetch(refspec, Some(&mut fetch_options), 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;
+    if let Ok(default_ref_buf) = remote.default_branch() {
+        if let Some(default_ref) = default_ref_buf.as_str() {
+            // LocalBranch here is the local branch on the remote, so it's really the remote
+            // branch
+            if let Some(RefName::LocalBranch(branch_name)) = parse_git_ref(default_ref) {
+                default_branch = Some(branch_name);
+            }
+        }
+    }
     import_refs(mut_repo, git_repo).map_err(|err| match err {
         GitImportError::InternalGitError(source) => GitFetchError::InternalGitError(source),
     })?;
-    Ok(())
+    Ok(default_branch)
 }
 
 #[derive(Error, Debug, PartialEq)]

src/commands.rs

@@ -3271,18 +3271,22 @@ fn cmd_git_clone(
     let remote_name = "origin";
     git_repo.remote(remote_name, source).unwrap();
     let mut tx = repo.start_transaction("fetch from git remote into empty repo");
-    git::fetch(tx.mut_repo(), &git_repo, remote_name).map_err(|err| match err {
-        GitFetchError::NoSuchRemote(_) => {
-            panic!("should't happen as we just created the git remote")
-        }
-        GitFetchError::InternalGitError(err) => {
-            CommandError::UserError(format!("Fetch failed: {:?}", err))
-        }
-    })?;
-    if let Ok(fetch_head_ref) = git_repo.find_reference("FETCH_HEAD") {
-        if let Ok(fetch_head_git_commit) = fetch_head_ref.peel_to_commit() {
-            let fetch_head_id = CommitId(fetch_head_git_commit.id().as_bytes().to_vec());
-            if let Ok(fetch_head_commit) = repo.store().get_commit(&fetch_head_id) {
+    let maybe_default_branch =
+        git::fetch(tx.mut_repo(), &git_repo, remote_name).map_err(|err| match err {
+            GitFetchError::NoSuchRemote(_) => {
+                panic!("should't happen as we just created the git remote")
+            }
+            GitFetchError::InternalGitError(err) => {
+                CommandError::UserError(format!("Fetch failed: {:?}", err))
+            }
+        })?;
+    if let Some(default_branch) = maybe_default_branch {
+        let default_branch_target = tx
+            .mut_repo()
+            .view()
+            .get_remote_branch(&default_branch, "origin");
+        if let Some(RefTarget::Normal(commit_id)) = default_branch_target {
+            if let Ok(fetch_head_commit) = repo.store().get_commit(&commit_id) {
                 tx.mut_repo().check_out(ui.settings(), &fetch_head_commit);
             }
         }