Add git merge rule (#755)

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

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)