Define RemoteConfig and RefSpec objects to parse remote blocks

Define RemoteConfig and RefSpec objects to parse remote blocks

To correctly handle all of the weird configuration rules within a
.git/config's remote section we need two entities: one describing
the remote, and one describing a lone ref specification ("src:dst"
pair).  The remote entity has to support more than one URL as Linus
long ago added the ability for C Git's push to upload to multiple
locations in a single command invocation.  The remote entity also
must support multiple pairs for the fetch and push specifications,
as users may configure more than one key in order to fetch (or push)
multiple branches.

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