fetch: Accept any SHA-1 on lhs of refspec

fetch: Accept any SHA-1 on lhs of refspec

Allow fetch to accept a SHA-1 on the left hand side of a RefSpec,
enabling callers to pass a specific SHA-1 they want that may not have
been advertised by the remote repository. This can be passed along to
the network protocol to be sent in a "want" line.

Rest of the plumbing only cares about the ObjectId of the Ref in
the askFor map, so make up a fake name using ObjectId.name() to
pass the desired ObjectId into the network code.

Change-Id: I620a189f3de005c403aa68b7d0442d6aa94e6056