fix: workspace_diff not resolving correctly when comparing against target branch that is moving or post-rebase

[Ziinc]

fixes bug where the workspace_diff post-autorebase does not resolve correctly:

Desired behaviour

  main ── U1 ── U2
             \
              W1   (workspace-authored commit)

  After force rebase, we still want the committed diff to reflect the workspace’s own commit lineage:

  main ── U1 ── U2
                 \
                  W1'  (rewritten workspace-authored commit)

buggy behaviour is such that the target branch (main) whcih now has U1 and U2, and the tip of the branch is resolved as @-, which resolves to W1 instead of W1' and hence the this results in Reviews tab showing "reversals" of commits (as we are now comparing in the reverse)