lfs: deduplicate oids in the transfer
Apparently, we can't rely on the server to deduplicate for us.
Sadly, the pointer object isn't hashable, so it can't be reduced by converting
it to a set. In order to be hashable, it needs to be immutable. I had a bunch
of code to change it to composition and forward the readonly dict methods to a
member dict. But the pointer is updated via __setitem__() when creating the
pointer file. So it didn't see worth adding all of that code to the class.