Simplify fetch transport API to just one function

Simplify fetch transport API to just one function

Commit walkers need to know the SHA-1 name of any objects they
have been asked to fetch while the native pack transport only
wants to know the names of the remote refs as the remote side
must do the name->SHA-1 translation.

Since we only have three fetch implementations and one of them
(bundle) doesn't even need the name information we can reduce
the code required to perform a fetch by having just one function
and passing of the filtered list of refs to be fetched.  Each
transport can then obtain the information it needs from that ref
array to construct its own internal operation state.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Conflicts:

	transport.c
Signed-off-by: Junio C Hamano <gitster@pobox.com>