Alembic operations fail with multiple head revisions

[cfm]

Description

All Alembic operations fail with Alembic error:

ERROR [alembic.util.messaging] Multiple head revisions are present for given argument 'head'; please specify a specific target revision, '<branchname>@head' to narrow to a specific head, or 'heads' for all heads

Cf. consistent recent failures of CI jobs app-tests and staging-test-with-rebase since #5974.

Steps to Reproduce

make test on develop; open or push to a PR; etc.

Expected Behavior

Alembic operations succeed and Alembic-based tests pass.

Actual Behavior

All Alembic operations and tests fail with Alembic error:

ERROR [alembic.util.messaging] Multiple head revisions are present for given argument 'head'; please specify a specific target revision, '<branchname>@head' to narrow to a specific head, or 'heads' for all heads

Comments

This is essentially an Alembic-level merge-conflict. PR forthcoming with the one-line fix.