Fix git hook checks all the staged files

I think because of this commit:

78e8165b06

The git hook will check all the staged files even it is not a Python source files.

This commit try to fix this and issue #271.

src/flake8/main/git.py

@@ -5,6 +5,7 @@
 
 """
 import contextlib
+import fnmatch
 import os
 import shutil
 import stat
@@ -40,7 +41,8 @@ def hook(lazy=False, strict=False):
     from flake8.main import application
     app = application.Application()
     with make_temporary_directory() as tempdir:
-        filepaths = list(copy_indexed_files_to(tempdir, lazy))
+        filepaths = [x for x in list(copy_indexed_files_to(tempdir, lazy))
+                     if fnmatch.fnmatch(x, '*.py')]
         app.initialize(['.'])
         app.options.exclude = update_excludes(app.options.exclude, tempdir)
         app.run_checks(filepaths)

tests/unit/test_git.py

@@ -27,3 +27,20 @@ def test_find_modified_files(lazy):
         git.find_modified_files(lazy)
 
     piped_process.assert_called_once_with(call)
+
+
+@mock.patch("flake8.main.git.copy_indexed_files_to")
+def test_only_py_files(mock_files):
+    """Confirm only run checks on Python source file."""
+    mock_files.return_value = [
+        "/tmp/xxx/test.py",
+        "/tmp/xxx/test.html",
+        "/tmp/xxx/test.txt",
+    ]
+
+    spec = "flake8.main.application.Application.run_checks"
+    with mock.patch(spec) as mock_run:
+        git.hook(lazy=False)
+        mock_run.assert_called_once_with([
+            "/tmp/xxx/test.py"
+        ])