Fix particle editmode undo not doing proper updates when child particles

are visible

Not freeing PTCacheEdit and tagging batch cache dirty on undo will have
a couple of consequences. This patch fixes:
- crash deleting a particle, then undo
- basically any edit (combing, ...), then undo will leave child hairs
untouched
- adding hairs (through mirror, add tool, ...), then undo will leave
'orphaned' child hairs

See also D5755 for a related discussion

Fixes the crasher mentioned in T69000

Might move this to a utility function later [since it is used in more
places], but that is for after going over some more reports...

Reviewers: sergey

Differential Revision: https://developer.blender.org/D5912

source/blender/editors/physics/particle_edit_undo.c

@@ -262,6 +262,11 @@ static void particle_undosys_step_decode(struct bContext *C,
   PTCacheEdit *edit = PE_get_current(depsgraph, scene, ob);
   if (edit) {
     undoptcache_to_editcache(&us->data, edit);
+    ParticleEditSettings *pset = &scene->toolsettings->particle;
+    if ((pset->flag & PE_DRAW_PART) != 0) {
+      psys_free_path_cache(NULL, edit);
+      BKE_particle_batch_cache_dirty_tag(edit->psys, BKE_PARTICLE_BATCH_DIRTY_ALL);
+    }
     DEG_id_tag_update(&ob->id, ID_RECALC_GEOMETRY);
   }
   else {