ENH(BM): add benchmark on feeding paths to subdasets call

not sure if it would be effective benchmark in this not so big
superdataset, but something at least

benchmarks/api.py

@@ -86,6 +86,12 @@ def time_ls_recursive_long_all(self):
     def time_subdatasets(self):
         self.ds.subdatasets()
 
+    def time_subdatasets_with_all_paths_recursive(self):
+        # to see if we do not get O(N^2) performance
+        subdatasets = self.ds.subdatasets(recursive=True, result_xfm='relpaths')
+        subdatasets2 = self.ds.subdatasets(path=subdatasets, recursive=True, result_xfm='relpaths')
+        assert subdatasets == subdatasets2
+
     def time_subdatasets_recursive(self):
         self.ds.subdatasets(recursive=True)