ftp: implement ls -s (show hashes)

ftp: implement ls -s (show hashes)

'bup ls' has a -s flag that can be used to show file hashes on the left
of each file name. 'bup ftp ls' doesn't have that feature.

Implement the feature by copying code from 'bup ls'. This is the last
feature difference between 'bup ls' and 'bup ftp ls' and bringing them
to the same level will make it possible to unify the code that is used
by both.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>