sha1-file: support OBJECT_INFO_FOR_PREFETCH

sha1-file: support OBJECT_INFO_FOR_PREFETCH

Teach oid_object_info_extended() to support a new flag that inhibits
fetching of missing objects. This is equivalent to setting
fetch_is_missing to 0, calling oid_object_info_extended(), then setting
fetch_if_missing to whatever it was before. Update unpack-trees.c to use
this new flag instead of repeatedly setting fetch_if_missing.

This new flag complicates things slightly in that there are now 2 ways
to do the same thing. But this eliminates the need to repeatedly set a
global variable, and more importantly, allows prefetching to be done in
parallel (in the future); hence, this patch.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>