Offer git commit --amend after previous git commit (#764)

tests/rules/test_git_commit_amend.py

@@ -0,0 +1,25 @@
+import pytest
+from thefuck.rules.git_commit_amend import match, get_new_command
+from thefuck.types import Command
+
+
+@pytest.mark.parametrize('script, output', [
+    ('git commit -m "test"', 'test output'),
+    ('git commit', '')])
+def test_match(output, script):
+    assert match(Command(script, output))
+
+
+@pytest.mark.parametrize('script', [
+    'git branch foo',
+    'git checkout feature/test_commit',
+    'git push'])
+def test_not_match(script):
+    assert not match(Command(script, ''))
+
+
+@pytest.mark.parametrize('script', [
+    ('git commit -m "test commit"'),
+    ('git commit')])
+def test_get_new_command(script):
+    assert get_new_command(Command(script, '')) == 'git commit --amend'