Fix #6357 - Don't remove useful signature files

duplicity >=0.6.20 now correctly removes useless signature files and
therefore the cache directory doesn't grow infinitely any more. Using
--extra-clean removes all signatures except from the last chain and so
renders older chains unusable, don't use it by default any more.
See: https://bugs.launchpad.net/duplicity/+bug/519948

handlers/dup.in

@@ -191,7 +191,9 @@ else
 fi
 
 ### Cleanup options
-execstr_options="${execstr_options} --extra-clean"
+if [ "$duplicity_major" -le 0 -a "$duplicity_minor" -le 6 -a "$duplicity_sub" -le 19 ]; then
+   execstr_options="${execstr_options} --extra-clean"
+fi
 
 ### Temporary directory
 if [ -n "$tmpdir" ]; then