Fix package update commit message to use correct template commit SHA

We previously read the template commit SHA from the package's
`.cruft.json` before actually performing the update. However, this
commit SHA wouldn't match the actually commit SHA of the actual
template commit when updating a package to a new template version.

This commit changes the field `PackageTemplater.template_commit` to a
property and reads the value from `.cruft.json` at the time the field is
read. This ensures that we always read the correct commit SHA when we
commit template changes in `package update`.

commodore/package/template.py

@@ -22,7 +22,6 @@
 class PackageTemplater(Templater):
     template_url: str
     template_version: str
-    template_commit: str
     _test_cases: list[str] = ["defaults"]
     copyright_year: Optional[str] = None
     _target_dir: Optional[Path] = None
@@ -43,8 +42,6 @@ def from_existing(cls, config: Config, package_path: Path):
         t.template_url = cruft_json["template"]
         if cruft_json["checkout"]:
             t.template_version = cruft_json["checkout"]
-        if cruft_json["commit"]:
-            t.template_commit = cruft_json["commit"]
 
         if "test_cases" in cookiecutter_args:
             t.test_cases = cookiecutter_args["test_cases"].split(" ")
@@ -54,6 +51,12 @@ def from_existing(cls, config: Config, package_path: Path):
         t.copyright_year = cookiecutter_args["copyright_year"]
         return t
 
+    @property
+    def template_commit(self) -> str:
+        with open(self.target_dir / ".cruft.json", "r", encoding="utf-8") as f:
+            cruft_json = json.load(f)
+            return cruft_json["commit"]
+
     @property
     def test_cases(self) -> list[str]:
         """Return list of test cases.

tests/test_package_template.py

@@ -298,6 +298,55 @@ def test_package_update_test_cases(
         assert r.repo.head.commit.message.startswith("Update from template\n\n")
 
 
+def test_package_update_commit_message(
+    tmp_path: Path, config: Config, cli_runner: RunnerFunc
+):
+    pkg_dir = tmp_path / "test-package"
+
+    # Intentionally create package from old version
+    result = cli_runner(
+        [
+            "-d",
+            str(tmp_path),
+            "package",
+            "new",
+            "test-package",
+            "--output-dir",
+            str(tmp_path),
+            "--template-version",
+            "main^",
+        ]
+    )
+    assert result.exit_code == 0
+
+    # Adjust cruft config to use "main" branch of template
+    with open(pkg_dir / ".cruft.json", "r", encoding="utf-8") as f:
+        cruft_json = json.load(f)
+        cruft_json["checkout"] = "main"
+    with open(pkg_dir / ".cruft.json", "w", encoding="utf-8") as f:
+        json.dump(cruft_json, f)
+
+    r = GitRepo(None, pkg_dir)
+    r.stage_files([".cruft.json"])
+    r.commit("Initial commit", amend=True)
+
+    # Update package
+    result = cli_runner(["-d", str(tmp_path), "package", "update", str(pkg_dir)])
+    print(result.stdout)
+    assert result.exit_code == 0
+
+    with open(pkg_dir / ".cruft.json", "r") as f:
+        cruft_json = json.load(f)
+        template_version = cruft_json["checkout"]
+        template_sha = cruft_json["commit"]
+
+    assert (
+        r.repo.head.commit.message
+        == "Update from template\n\n"
+        + f"Template version: {template_version} ({template_sha[:7]})"
+    )
+
+
 def test_package_templater_from_existing_nonexistent(tmp_path: Path, config: Config):
     with pytest.raises(click.ClickException) as e:
         _ = PackageTemplater.from_existing(config, tmp_path / "test-package")