add(rule): add the new git_diff_staged rule

README.md

@@ -159,6 +159,7 @@ using the matched rule and runs it. Rules enabled by default are as follows:
 * `java` – removes `.java` extension when running Java programs;
 * `git_add` – fix *"Did you forget to 'git add'?"*;
 * `git_checkout` – creates the branch before checking-out;
+* `git_diff_staged` – adds `--staged` to previous `git diff` with unexpected output;
 * `git_no_command` – fixes wrong git commands like `git brnch`;
 * `git_pull` – sets upstream before executing previous `git pull`;
 * `git_push` – adds `--set-upstream origin $branch` to previous failed `git push`;

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

thefuck/rules/git_diff_staged.py

@@ -0,0 +1,6 @@
+def match(command, settings):
+    return command.script.startswith('git d')
+
+
+def get_new_command(command, settings):
+    return '{} --staged'.format(command.script)