Remove old placeholder for merge-head
Remove graph/ qualifiers for names in merge test
Fix mistake: two functions were comp'd in the wrong order
Start of a merge/unmerge test for a borg tree
Fix silly bug in leaf-seq's recursion
Don't look up first-merged unless it will be needed
Fix merge-point - it should start at tail's parent, not at tail
Fix ruminate-merge - it should look for roots of child, not of tail-id.

Because tail-id still thinks it's connected to the old head.
Make sure to use appending, not overwriting, layers in merge tests
Simplify a test helper
A bunch more assertions in multiple-merges test
Make merge layer propagate :position to its incoming
Add tests for unmerging to-ids of edges
Test that incoming edges work on merged layers
Ported three important merge tests from old merge layer
Add notes about which merge tests we still want to port