Upsert tags in two queries

Before, we had an N+1 query, where each tag would be separately fetched
and updated. Instead, first delete any tags we no longer care about,
then perform an upsert of all fields except for `last_commit`, which we
want to keep from the existing tag.

This comes with one caveat: on the first update of the tags, the `tags`
list is no longer modified in-place, because mutation is bad, however,
the database state is updated accordingly.

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