Mark the list of refs to fetch as const

Mark the list of refs to fetch as const

Fetching the objects doesn't actually modify the list in any of the
code paths, so this will allow code that fetches the entire (const)
list of available refs to just pass the list in directly.

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