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.

README.md

@@ -186,6 +186,7 @@ using the matched rule and runs it. Rules enabled by default are as follows:
 * `git_rebase_no_changes` – runs `git rebase --skip` instead of `git rebase --continue` when there are no changes;
 * `git_rm_local_modifications` –  adds `-f` or `--cached` when you try to `rm` a locally modified file;
 * `git_rm_recursive` – adds `-r` when you try to `rm` a directory;
+* `git_rm_staged` –  adds `-f` or `--cached` when you try to `rm` a file with staged changes
 * `git_rebase_merge_dir` – offers `git rebase (--continue | --abort | --skip)` or removing the `.git/rebase-merge` dir when a rebase is in progress;
 * `git_remote_seturl_add` – runs `git remote add` when `git remote set_url` on nonexistant remote;
 * `git_stash` – stashes your local modifications before rebasing or switching branch;

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

thefuck/rules/git_rm_staged.py

@@ -0,0 +1,19 @@
+from thefuck.specific.git import git_support
+
+
+@git_support
+def match(command):
+    return (' rm ' in command.script and
+            'error: the following file has changes staged in the index' in command.stderr and
+            'use --cached to keep the file, or -f to force removal' in command.stderr)
+
+
+@git_support
+def get_new_command(command):
+    command_parts = command.script_parts[:]
+    index = command_parts.index('rm') + 1
+    command_parts.insert(index, '--cached')
+    command_list = [u' '.join(command_parts)]
+    command_parts[index] = '-f'
+    command_list.append(u' '.join(command_parts))
+    return command_list