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.c

@@ -27,7 +27,7 @@ void put_be16(uint8_t *out, uint16_t i)
 	out[1] = (uint8_t)(i & 0xff);
 }
 
-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)
 {
 	size_t lo = 0;
 	size_t hi = sz;