remote-helpers: Fetch more than one ref in a batch

remote-helpers: Fetch more than one ref in a batch

Some network protocols (e.g. native git://) are able to fetch more
than one ref at a time and reduce the overall transfer cost by
combining the requests into a single exchange.  Instead of feeding
each fetch request one at a time to the helper, feed all of them
at once so the helper can decide whether or not it should batch them.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>