FirstWant: Parse client session-id if received.

FirstWant: Parse client session-id if received.

In protocol V0 the client capabilities are appended to the first line.
Parsing session-id is currently only supported during a ReceivePack
operation. This change will parse the client session-id capability if
it has been sent by the client.

If the server sends the session-id capability to the client. The client
may respond with a session ID of its own. FirstWant.fromLine will now
parse the ID and make it available via the getClientSID method.

This change does not add support to send the session-id capability from
the server. The change is necessary to support session-id in UploadPack.

Change-Id: Id3fe44fdf9a72984ee3de9cf40cc4e71d434df4a
Signed-off-by: Josh Brown <sjoshbrown@google.com>