Added clean_state_files

pygit2/decl.h

@@ -114,6 +114,7 @@ int git_remote_create(
 	git_repository *repo,
 	const char *name,
 	const char *url);
+int git_repository_state_cleanup(git_repository *repo);
 
 const char * git_remote_name(const git_remote *remote);
 

pygit2/repository.py

@@ -299,3 +299,6 @@ class Repository(_Repository):
             raise NotImplementedError('git_diff_blob_to_blob()')
 
         raise ValueError("Only blobs and treeish can be diffed")
+
+    def clean_state_files(self):
+        C.git_repository_state_cleanup(self._repo)