Unlike its twenty siblings it ran a bare add_column plus create_index, so
`flask plugin upgrade-all` failed with "duplicate column name: gagelabtag" on
any database whose tables were built by create_all and stamped nowhere - and a
chain stops at its first error, so 0004 never applied either.
The index is guarded by COLUMN SET rather than by name: create_all names indexes
by SQLAlchemy's convention and the migration by this repo's, so a name-keyed
guard would have added a second index over the same column instead of skipping.
The test that should have caught it was restricted to plugins whose whole chain
is a single baseline, on the reasoning that a longer chain meets a second
problem - an ALTER whose column is already present - that guarding a create_table
does not fix. True, but the conclusion was wrong: the answer is to guard the
ALTERs too, not to stop testing the chains that have them. It now covers every
post-cutover chain, and was confirmed to fail against the unguarded migration
before the guard went in.
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