Add git merge rule (#755)

This fixes https://github.com/nvbn/thefuck/issues/629

tests/rules/test_git_merge.py

@@ -0,0 +1,26 @@
+import pytest
+from thefuck.rules.git_merge import match, get_new_command
+from thefuck.types import Command
+
+
+@pytest.fixture
+def output():
+    return 'merge: local - not something we can merge\n\n' \
+        'Did you mean this?\n\tremote/local'
+
+
+def test_match(output):
+    assert match(Command('git merge test', output))
+    assert not match(Command('git merge master', ''))
+    assert not match(Command('ls', output))
+
+
+@pytest.mark.parametrize('command, new_command', [
+    (Command('git merge local', output()),
+     'git merge remote/local'),
+    (Command('git merge -m "test" local', output()),
+     'git merge -m "test" remote/local'),
+    (Command('git merge -m "test local" local', output()),
+     'git merge -m "test local" remote/local')])
+def test_get_new_command(command, new_command):
+    assert get_new_command(command) == new_command