fetch-pack: sort the list of incoming refs

fetch-pack: sort the list of incoming refs

Having the list sorted means we can avoid some quadratic
algorithms when comparing lists.

These should typically be sorted already, but they do come
from the remote, so let's be extra careful. Our ref-sorting
implementation does a mergesort, so we do not have to care
about performance degrading in the common case that the list
is already sorted.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>