object: correct the expected ID size in prefix lookup

We take in a possibly partial ID by taking a length and working off of
that to figure out whether to just look up the object or ask the
backends for a prefix lookup.

Unfortunately we've been checking the size against `GIT_OID_HEXSZ` which
is the size of a *string* containing a full ID, whereas we need to check
against the size we can have when it's a 20-byte array.

Change the checks and comment to use `GIT_OID_RAWSZ` which is the
correct size of a git_oid to have when full.

src/object.c

@@ -129,10 +129,10 @@ int git_object_lookup_prefix(
 	if (error < 0)
 		return error;
 
-	if (len > GIT_OID_HEXSZ)
-		len = GIT_OID_HEXSZ;
+	if (len > GIT_OID_RAWSZ)
+		len = GIT_OID_RAWSZ;
 
-	if (len == GIT_OID_HEXSZ) {
+	if (len == GIT_OID_RAWSZ) {
 		git_cached_obj *cached = NULL;
 
 		/* We want to match the full id : we can first look up in the cache,
@@ -172,9 +172,9 @@ int git_object_lookup_prefix(
 		memcpy(short_oid.id, id->id, (len + 1) / 2);
 		if (len % 2)
 			short_oid.id[len / 2] &= 0xF0;
-		memset(short_oid.id + (len + 1) / 2, 0, (GIT_OID_HEXSZ - len) / 2);
+		memset(short_oid.id + (len + 1) / 2, 0, (GIT_OID_RAWSZ - len) / 2);
 
-		/* If len < GIT_OID_HEXSZ (a strict short oid was given), we have
+		/* If len < GIT_OID_RAWSZ (a strict short oid was given), we have
 		 * 2 options :
 		 * - We always search in the cache first. If we find that short oid is
 		 *	ambiguous, we can stop. But in all the other cases, we must then