reftable/basics: adjust hash_size() to return uint32_t

The `hash_size()` function returns the number of bytes used by the hash
function. Weirdly enough though, it returns a signed integer for its
size even though the size obviously cannot ever be negative. The only
case where it could be negative is if the function returned an error
when asked for an unknown hash, but we assert(3p) instead.

Adjust the type of `hash_size()` to be `uint32_t` and adapt all places
that use signed integers for the hash size to follow suit. This also
allows us to get rid of a couple asserts that we had which verified that
the size was indeed positive, which further stresses the point that this
refactoring makes sense.

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

reftable/basics.c

@@ -272,7 +272,7 @@ size_t common_prefix_size(struct reftable_buf *a, struct reftable_buf *b)
 	return p;
 }
 
-int hash_size(enum reftable_hash id)
+uint32_t hash_size(enum reftable_hash id)
 {
 	if (!id)
 		return REFTABLE_HASH_SIZE_SHA1;