add(rule): add the new git_diff_staged rule

tests/rules/test_git_diff_staged.py

@@ -0,0 +1,27 @@
+import pytest
+from thefuck.rules.git_diff_staged import match, get_new_command
+from tests.utils import Command
+
+
+@pytest.mark.parametrize('command', [
+    Command(script='git diff'),
+    Command(script='git df'),
+    Command(script='git ds')])
+def test_match(command):
+    assert match(command, None)
+
+
+@pytest.mark.parametrize('command', [
+    Command(script='git tag'),
+    Command(script='git branch'),
+    Command(script='git log')])
+def test_not_match(command):
+    assert not match(command, None)
+
+
+@pytest.mark.parametrize('command, new_command', [
+    (Command('git diff'), 'git diff --staged'),
+    (Command('git df'), 'git df --staged'),
+    (Command('git ds'), 'git ds --staged')])
+def test_get_new_command(command, new_command):
+    assert get_new_command(command, None) == new_command