reftable/stack: use size_t to track stack length

While the stack length is already stored as `size_t`, we frequently use
`int`s to refer to those stacks throughout the reftable library. Convert
those cases to use `size_t` instead to make things consistent.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/basics.h

@@ -44,7 +44,7 @@ void parse_names(char *buf, int size, char ***namesp);
 int names_equal(char **a, char **b);
 
 /* returns the array size of a NULL-terminated array of strings. */
-int names_length(char **names);
+size_t names_length(char **names);
 
 /* Allocation routines; they invoke the functions set through
  * reftable_set_alloc() */