follow-up on but-graph updates

[Byron]

Follow-up on #13821.

TBD

Tasks

• The traversal could auto-pick target refs if unset to fill in a target most of the time, including one read from a global ref like GITBUTLER_TARGET.
  • If so, have an example but version that works in SBM automatically as long as there is a remote. If not, it
• auto-set the target ref in single-branch mode according to the new discovery rules (remote/HEAD, initial branch name, never the same as the current branch, communicated through tips_of_interest)
• Use target branch tip as base for new independent branches #13626
  • use it's test-case (presuming it is what I think it should be) as case for adding the highest-base to points of interest.
  • Show that we pick up virtual branches at the highest and lowest base
  • Add an option to create reference so it can add it at the highest or lowest base respectively.
• Prune integrated commits at or below the target from all stacks #13627
• Use merge-base override to prevent consumed changes reappearing as uncommitted #13546
• Factor change similarity to low level function #13620
  • deduplicate and offer a way to attach more data Graph. This way, the per-commit enrichment can be shared in ctx
• TODO: about Fix workspace projection pruning for integrated bottom segments #13552