GroupNesting Iterator skips sibling groups

I couldn't find anywhere in core that actually uses the nesting iterator, but I noticed it not behaving correctly when I used it in a custom module.

The purpose of the iterator is to list groups in order of the nesting tree, but the iterator does not check for siblings appropriately. For example, with a group structure like the following:

Group 1
– Group 1_1
– – Group 1_1_1
– Group 1_2 –
– Group 1_3
– – Group 1_3_1
– – Group 1_3_2
Group 2
– Group 2_1
– Group 2_2

The iterator will actually put something like the following out, skipping sibling groups as it moves back up the tree.
Group 1
– Group 1_1
– – Group 1_1_1
Group 2
– Group 2_1

The attached patch should fix the issue. Also, why i was in there, I made the indent separator configurable, though it will still default to  --