CmdPurge: Require GC run prior to execution

Since TDB2::purge method allows purging of the tasks only
on competed.data file, we need to make sure that all the tasks
that were supposed to be moved to completed.data have been
moved - in other words, we need to run GC.

This allows for diract usage of purge after delete, that is:

$ task <filter> delete
$ task <filter> purge

working as expected.

src/commands/CmdPurge.cpp

@@ -42,7 +42,7 @@ CmdPurge::CmdPurge ()
   _read_only             = false;
   _displays_id           = false;
   _needs_confirm         = true;
-  _needs_gc              = false;
+  _needs_gc              = true;
   _uses_context          = true;
   _accepts_filter        = true;
   _accepts_modifications = false;