cli: when initializing repo backed by Git repo, check out Git's HEAD

Closes #102.

src/commands.rs

@@ -1757,6 +1757,14 @@ fn cmd_init(ui: &mut Ui, command: &CommandHelper, args: &ArgMatches) -> Result<(
         }
         let mut tx = workspace_command.start_transaction("import git refs");
         git::import_refs(tx.mut_repo(), &git_repo)?;
+        if let Some(git_head_id) = tx.mut_repo().view().git_head() {
+            let git_head_commit = tx.mut_repo().store().get_commit(&git_head_id)?;
+            tx.mut_repo().check_out(
+                workspace_command.workspace_id(),
+                ui.settings(),
+                &git_head_commit,
+            );
+        }
         // TODO: Check out a recent commit. Maybe one with the highest generation
         // number.
         if tx.mut_repo().has_changes() {

tests/test_init_command.rs

@@ -43,7 +43,31 @@ fn test_init_git_internal() {
 fn test_init_git_external() {
     let test_env = TestEnvironment::default();
     let git_repo_path = test_env.env_root().join("git-repo");
-    git2::Repository::init(&git_repo_path).unwrap();
+    let git_repo = git2::Repository::init(&git_repo_path).unwrap();
+    let git_blob_oid = git_repo.blob(b"some content").unwrap();
+    let mut git_tree_builder = git_repo.treebuilder(None).unwrap();
+    git_tree_builder
+        .insert("some-file", git_blob_oid, 0o100644)
+        .unwrap();
+    let git_tree_id = git_tree_builder.write().unwrap();
+    let git_tree = git_repo.find_tree(git_tree_id).unwrap();
+    let git_signature = git2::Signature::new(
+        "Git User",
+        "[email protected]",
+        &git2::Time::new(123, 60),
+    )
+    .unwrap();
+    git_repo
+        .commit(
+            Some("refs/heads/my-branch"),
+            &git_signature,
+            &git_signature,
+            "My commit message",
+            &git_tree,
+            &[],
+        )
+        .unwrap();
+    git_repo.set_head("refs/heads/my-branch").unwrap();
 
     let assert = test_env
         .jj_cmd(
@@ -57,8 +81,11 @@ fn test_init_git_external() {
         )
         .assert()
         .success();
-    insta::assert_snapshot!(get_stdout_string(&assert), @r###"Initialized repo in "repo"
-"###);
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    Working copy now at: f6950fc115ae 
+    Added 1 files, modified 0 files, removed 0 files
+    Initialized repo in "repo"
+    "###);
 
     let workspace_root = test_env.env_root().join("repo");
     let jj_path = workspace_root.join(".jj");
@@ -73,6 +100,16 @@ fn test_init_git_external() {
     assert!(git_target_file_contents
         .replace('\\', "/")
         .ends_with("/git-repo/.git"));
+
+    // Check that the Git repo's HEAD got checked out
+    let assert = test_env
+        .jj_cmd(&repo_path, &["log", "-r", "@-"])
+        .assert()
+        .success();
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    o 8d698d4a8ee1 d3866db7e30a [email protected] 1970-01-01 01:02:03.000 +01:00 my-branch   HEAD@git
+    ~ My commit message
+    "###);
 }
 
 #[test]