Re: [PATCH 2/5] streaming.c: remove enum/function/vtbl indirection

> Remove the indirection of discovering a function pointer to use via an
> enum and virtual table. This refactors code added in
> 46bf043807c (streaming: a new API to read from the object store,
> 2011-05-11).
> 
> We can instead simply return an "open_istream_fn" for use from the
> "istream_source()" selector function directly. This allows us to get
> rid of the "incore", "loose" and "pack_non_delta" enum
> variables. We'll return the functions instead.
> 
> The "stream_error" variable in that enum can likewise go in favor of
> returning NULL, which is what the open_istream() was doing when it got
> that value anyway.
> 
> We can thus remove the entire enum, and the "open_istream_tbl" virtual
> table that (indirectly) referenced it.