refs.c: add a function to sort a ref list, rather then sorting on add

refs.c: add a function to sort a ref list, rather then sorting on add

Rather than sorting the refs list while building it, sort in one
go after it is built using a merge sort.  This has a large
performance boost with large numbers of refs.

It shouldn't happen that we read duplicate entries into the same
list, but just in case sort_ref_list drops them if the SHA1s are
the same, or dies, as we have no way of knowing which one is the
correct one.

Signed-off-by: Julian Phillips <redacted>
Acked-by: Linus Torvalds <redacted>
Signed-off-by: Junio C Hamano <redacted>