revisit decision to assume nodes following delta-merge must be merged too

As part of HCM-93, the migration tool was adjusted to support same-name siblings. One assumption we made is that nodes without a esv:merge attribute following a delta-merge can always be merged too. This is different behavior as in 11.x, where nodes without a esv:merge attribute would result in a new same-name sibling. It was expected that there are few cases where that difference is significant in project configuration.

We also discussed the possibility to create upgrade tooling to detect unexpected differences between the 11.x esv bootstrap result and the 12.0 yaml bootstrap result.