delta: extend skip logic to work on full directory paths (prefix+suffix)

delta: extend skip logic to work on full directory paths (prefix+suffix)

Rather than only checking that a prefix path is a symlink to another prefix
path (this can happen with split-usr=true and /lib a symlink to /usr/lib),
extend the logic in should_skip_prefix() so that the check is done on the full
directory path (prefix + suffix).

This allows to catch such cases as well:

 # ls -ld /lib /lib/udev
 drwxr-xr-x 1 root root 86 Nov 22 13:14 /lib
 lrwxrwxrwx 1 root root 13 Sep  4 17:47 /lib/udev -> /usr/lib/udev

where prefix=/lib and suffix=udev/rules.d