Add git_merge_unrelated rule for git merge --allow-unrelated-histories (#773)

From https://git-scm.com/docs/merge-options#merge-options---allow-unrelated-histories

> By default, `git merge` command refuses to merge histories that do not
share a common ancestor. This option can be used to override this safety
when merging histories of two projects that started their lives
independently.

tests/rules/test_git_merge_unrelated.py

@@ -0,0 +1,25 @@
+import pytest
+from thefuck.rules.git_merge_unrelated import match, get_new_command
+from thefuck.types import Command
+
+
+@pytest.fixture
+def output():
+    return 'fatal: refusing to merge unrelated histories'
+
+
+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 local --allow-unrelated-histories'),
+    (Command('git merge -m "test" local', output()),
+     'git merge -m "test" local --allow-unrelated-histories'),
+    (Command('git merge -m "test local" local', output()),
+     'git merge -m "test local" local --allow-unrelated-histories')])
+def test_get_new_command(command, new_command):
+    assert get_new_command(command) == new_command