refs: Use binary search to lookup refs faster

Currently we linearly search through lists of refs when we need to
find a specific ref.  This can be very slow if we need to lookup a
large number of refs.  By changing to a binary search we can make this
faster.

In order to be able to use a binary search we need to change from
using linked lists to arrays, which we can manage using ALLOC_GROW.

We can now also use the standard library qsort function to sort the
refs arrays.

Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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