rcup: Fix shell quoting in is_linked

This resulted in spurious "Replacing identical but unlinked ..."
messages for destinations with spaces in their names, like macOS's
`~/Library/Application Support` directory.

Author: 9999years

bin/rcup.in

@@ -130,7 +130,7 @@ is_linked() {
   local dest="$2"
 
   if [ -h "$dest" ]; then
-    local link_dest=$(readlink $dest)
+    local link_dest=$(readlink "$dest")
     [ "$link_dest" = "$src" ]
   else
     return 1