Currently ref lookups require loading the entire packed-refs file into
a hashmap in memory. For repos with large numbers of refs this can be
painfully slow.

This patch replaces the existing lookup code and instead mmap()'s the
packed-refs file and performs a binary search to locate the ref entry.
Git uses a similiar approach.

The old hash table codepath is still used for unsorted packed-refs files.

This patch also fixes a minor bug where the "peeled" trait is never
parsed correctly from the packed-refs header.