Bitops: Stop overallocating storage space on set

Previously the string was created empty then re-sized
to fit the offset, but sds resize causes the sds to
over-allocate by at least 1 MB (which is a lot when
you are operating at bit-level access).

This also improves the speed of initial sets by 2% to 6%
based on quick testing.

Patch logic provided by @oranagra

Fixes #1918

src/t_string.c

@@ -194,7 +194,7 @@ void setrangeCommand(redisClient *c) {
         if (checkStringLength(c,offset+sdslen(value)) != REDIS_OK)
             return;
 
-        o = createObject(REDIS_STRING,sdsempty());
+        o = createObject(REDIS_STRING,sdsnewlen(NULL, offset+sdslen(value)));
         dbAdd(c->db,c->argv[1],o);
     } else {
         size_t olen;