Blender Git Loki

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