Add git_rm_staged rule for removing locally staged changes

It would be nice if `thefuck` could help me `git rm` a file I had
already staged. This rule, adapted from `git_rm_local_modifications`,
does that.

tests/rules/test_git_rm_staged.py

@@ -0,0 +1,28 @@
+import pytest
+from thefuck.rules.git_rm_staged import match, get_new_command
+from tests.utils import Command
+
+
+@pytest.fixture
+def stderr(target):
+    return ('error: the following file has changes staged in the index:\n    {}\n(use '
+            '--cached to keep the file, or -f to force removal)').format(target)
+
+
+@pytest.mark.parametrize('script, target', [
+    ('git rm foo', 'foo'),
+    ('git rm foo bar', 'bar')])
+def test_match(stderr, script, target):
+    assert match(Command(script=script, stderr=stderr))
+
+
+@pytest.mark.parametrize('script', ['git rm foo', 'git rm foo bar', 'git rm'])
+def test_not_match(script):
+    assert not match(Command(script=script, stderr=''))
+
+
+@pytest.mark.parametrize('script, target, new_command', [
+    ('git rm foo', 'foo', ['git rm --cached foo', 'git rm -f foo']),
+    ('git rm foo bar', 'bar', ['git rm --cached foo bar', 'git rm -f foo bar'])])
+def test_get_new_command(stderr, script, target, new_command):
+    assert get_new_command(Command(script=script, stderr=stderr)) == new_command