Here we define each file set as those found by a recursive search of a
single directory argument to btrfs fi du.

This isn't as simple as adding up shared extents - they may be shared with
each other, and may also overlap. This patch uses an interval tree to store
shared extents we find while fiemapping files. After collecting them, a 'set
shared' count is calculated by summing (without overlap) each shared region
discovered. This is then displayed to the user as 'set shared'.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>