Add git merge rule (#755)

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

README.md

@@ -191,6 +191,7 @@ using the matched rule and runs it. Rules enabled by default are as follows:
 * `git_fix_stash` – fixes `git stash` commands (misspelled subcommand and missing `save`);
 * `git_flag_after_filename` – fixes `fatal: bad flag '...' after filename`
 * `git_help_aliased` &ndash; fixes `git help <alias>` commands replacing <alias> with the aliased command;
+* `git_merge` &ndash; adds remote to branch names;
 * `git_not_command` &ndash; fixes wrong git commands like `git brnch`;
 * `git_pull` &ndash; sets upstream before executing previous `git pull`;
 * `git_pull_clone` &ndash; clones instead of pulling when the repo does not exist;

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

thefuck/rules/git_merge.py

@@ -0,0 +1,18 @@
+import re
+from thefuck.utils import replace_argument
+from thefuck.specific.git import git_support
+
+
+@git_support
+def match(command):
+    return ('merge' in command.script
+            and ' - not something we can merge' in command.output
+            and 'Did you mean this?' in command.output)
+
+
+@git_support
+def get_new_command(command):
+    unknown_branch = re.findall(r'merge: (.+) - not something we can merge', command.output)[0]
+    remote_branch = re.findall(r'Did you mean this\?\n\t([^\n]+)', command.output)[0]
+
+    return replace_argument(command.script, unknown_branch, remote_branch)