reftable/basics: fix return type of binsearch() to be size_t

The `binsearch()` function can be used to find the first element for
which a callback functions returns a truish value. But while the array
size is of type `size_t`, the function in fact returns an `int` that is
supposed to index into that array.

Fix the function signature to return a `size_t`. This conversion does
not change any semantics given that the function would only ever return
a value in the range `[0, sz]` anyway.

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

reftable/basics.h

@@ -28,7 +28,7 @@ void put_be16(uint8_t *out, uint16_t i);
  * Contrary to bsearch(3), this returns something useful if the argument is not
  * found.
  */
-int binsearch(size_t sz, int (*f)(size_t k, void *args), void *args);
+size_t binsearch(size_t sz, int (*f)(size_t k, void *args), void *args);
 
 /*
  * Frees a NULL terminated array of malloced strings. The array itself is also