Remember pre-existing trip expansion state when autogrouping

This just adds the "remember_tree_state()" and "restore_tree_state()"
calls around the divelist update when autogrouping is turned on: that
way existing expanded groups aren't blindly collapsed.

It makes it much easier to see what autogrouping does if it only groups
a few dives, because the overall state of the divelist isn't radically
changed.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>