tryObjectEncoding(): optimize sds strings if possible.

When no encoding is possible, at least try to reallocate the sds string
with one that does not waste memory (with free space at the end of the
buffer) when the string is large enough.

src/object.c

@@ -363,7 +363,22 @@ robj *tryObjectEncoding(robj *o) {
             decrRefCount(o);
             return emb;
         } else {
-            /* Otherwise return the original object. */
+            /* We can't encode the object...
+             *
+             * Do the last try, and at least optimize the SDS string inside
+             * the string object to require little space, in case there
+             * is more than 10% of free space at the end of the SDS string.
+             *
+             * We do that only for relatively large strings as this branch
+             * is only entered if the length of the string is greater than
+             * REDIS_ENCODING_EMBSTR_SIZE_LIMIT. */
+            if (len > 64 &&
+                o->encoding == REDIS_ENCODING_RAW &&
+                sdsavail(s) > len/10)
+            {
+                o->ptr = sdsRemoveFreeSpace(o->ptr);
+            }
+            /* Return the original object. */
             return o;
         }
     }