Fix stable sort when reverse option is enabled

[joelim-work]

• Fixes Reverse command works incorrectly #901

The dir.sort function uses stable sorting so that the relative order of equivalent elements is preserved. The problem is that if the reverse option is enabled, then the order will be reversed afterwards, so ultimately the order of equivalent elements will be reversed.

To reproduce this, just use set reverse and press ss repeatedly, and the directories will reverse their order since the directories themselves generally have the same size.

[gokcehan]

@joelim-work Looks good, thanks for the patch.