update_refs(): fix constness

update_refs(): fix constness

The old signature of update_refs() required a
(const struct ref_update **) for its updates_orig argument.  The
"const" is presumably there to promise that the function will not
modify the contents of the structures.

But this declaration does not permit the function to be called with a
(struct ref_update **), which is perfectly legitimate.  C's type
system is not powerful enough to express what we'd like.  So remove
the first "const" from the declaration.

On the other hand, the function *can* promise not to modify the
pointers within the array that is passed to it without inconveniencing
its callers.  So add a "const" that has that effect, making the final
declaration
(struct ref_update * const *).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>