test: add missing tests for get_filenames_in_commit with git_reference

Add test coverage for get_filenames_in_commit() when called with an
explicit git_reference parameter. Previously only the error case and
the integration test (no git_reference) existed.

New tests:
- test_get_filenames_in_commit: success case without git_reference
- test_get_filenames_in_commit_with_git_reference: success case with
  explicit reference, verifying the correct git command is called
- test_get_filenames_in_commit_error_with_git_reference: error case
  with explicit reference

Closes #1860

Signed-off-by: edvatar <88481784+toroleapinc@users.noreply.github.com>

Author: toroleapinc

tests/test_git.py

@@ -387,6 +387,21 @@ def test_commit_with_spaces_in_path(
     mock_unlink.assert_called_once_with(file_path)
 
 
+def test_get_filenames_in_commit(util: UtilFixture):
+    """Test getting filenames from the last commit (default, no git_reference)."""
+    util.mock_cmd(out="file1.py\nfile2.py\n")
+    result = git.get_filenames_in_commit()
+    assert result == ["file1.py", "file2.py"]
+
+
+def test_get_filenames_in_commit_with_git_reference(util: UtilFixture):
+    """Test getting filenames with an explicit git reference."""
+    mock = util.mock_cmd(out="src/main.py\nREADME.md\n")
+    result = git.get_filenames_in_commit("HEAD~1")
+    assert result == ["src/main.py", "README.md"]
+    mock.assert_called_once_with("git show --name-only --pretty=format: HEAD~1")
+
+
 def test_get_filenames_in_commit_error(util: UtilFixture):
     """Test that GitCommandError is raised when git command fails."""
     util.mock_cmd(err="fatal: bad object HEAD", return_code=1)
@@ -395,6 +410,17 @@ def test_get_filenames_in_commit_error(util: UtilFixture):
     assert str(excinfo.value) == "fatal: bad object HEAD"
 
 
+def test_get_filenames_in_commit_error_with_git_reference(util: UtilFixture):
+    """Test that GitCommandError is raised with an explicit git reference."""
+    mock = util.mock_cmd(
+        err="fatal: bad object abc123", return_code=128
+    )
+    with pytest.raises(GitCommandError) as excinfo:
+        git.get_filenames_in_commit("abc123")
+    assert str(excinfo.value) == "fatal: bad object abc123"
+    mock.assert_called_once_with("git show --name-only --pretty=format: abc123")
+
+
 @pytest.mark.parametrize(
     "linebreak", ["\n", "\r\n"], ids=["line_feed", "carriage_return"]
 )