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

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>