util: whitelist apfs for hardlink support

Seems to work fine for me.

Differential Revision: https://phab.mercurial-scm.org/D3134

mercurial/util.py

@@ -1544,6 +1544,7 b' def checksignature(func):'
 
 # a whilelist of known filesystems where hardlink works reliably
 _hardlinkfswhitelist = {
+    'apfs',
     'btrfs',
     'ext2',
     'ext3',