btrbk: bugfix: do not keep latest common snapshot for "incremental no"

There is no reason to keep the latest common source snapshot if
"incremental no" is configured (see #252).

ChangeLog

@@ -11,6 +11,8 @@ btrbk-0.27.0-dev
   * Bugfix: add "btrfs filesystem usage" backend mapping (close #222).
   * Bugfix: do not fail (exitcode=10) if targets are not accessible on
     "btrfs snapshot --preserve".
+  * Bugfix: if "incremental no" is configured on target, do not keep
+    latest common snapshot.
   * Enhance internal data structures:
     - Allow snapshot_dir to be a mountpoint.
     - Search complete target tree for correlated subvolumes.

btrbk

@@ -5990,7 +5990,8 @@ MAIN:
               next;
             }
 
-            # always preserve latest common snapshot/backup pair
+            # preserve latest common snapshot/backup (for incremental targets)
+            if(config_key($droot, "incremental")) {
               foreach my $child (@snapshot_children) {
                 my @receive_targets = get_receive_targets($droot, $child, exact => 1);
                 if(scalar(@receive_targets)) {
@@ -6003,6 +6004,7 @@ MAIN:
                   last;
                 }
               }
+            }
 
             if($preserve_backups) {
               INFO "Preserving all backups";