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

@@ -293,7 +293,26 @@ robj *tryObjectEncoding(robj *o) {
 
     /* Check if we can represent this string as a long integer */
     len = sdslen(s);
-    if (len > 21 || !string2l(s,len,&value)) return o;
+    if (len > 21 || !string2l(s,len,&value)) {
+        /* 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 for larger strings, using the arbitrary value
+         * of 32 bytes. This code was backported from the unstable branch
+         * where this is performed when the object is too large to be
+         * encoded as EMBSTR. */
+        if (len > 32 &&
+            o->encoding == REDIS_ENCODING_RAW &&
+            sdsavail(s) > len/10)
+        {
+            o->ptr = sdsRemoveFreeSpace(o->ptr);
+        }
+        /* Return the original object. */
+        return o;
+    }
 
     /* Ok, this object can be encoded...
      *