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.

thefuck/rules/git_merge_unrelated.py

@@ -0,0 +1,12 @@
+from thefuck.specific.git import git_support
+
+
+@git_support
+def match(command):
+    return ('merge' in command.script
+            and 'fatal: refusing to merge unrelated histories' in command.output)
+
+
+@git_support
+def get_new_command(command):
+    return command.script + ' --allow-unrelated-histories'