Ensure that shipping the item _commit fix doesn't break existing databases

[simonw]

An interesting challenge with this change: since it modifies the schema, shipping a release with it could break existing databases the next time git-history file ... is run against them.

This would affect my workflow here for example: https://github.com/simonw/scrape-instances-social/blob/main/.github/workflows/scrape.yml

Originally posted by @simonw in #59 (comment)

[simonw]

Worth noting that this really was a bug: the code was designed to create that _commit column but failed to because it mutated a copy of the item from the array, not the original object:

git-history/git_history/cli.py

Lines 244 to 252 in 91abda0

	for item in items:
	item = jsonify_all(fix_reserved_columns(item))
	item["_commit"] = commit_pk
	db[item_table].insert_all(
	items,
	column_order=("_id",),
	alter=True,
	foreign_keys=(("_commit", "commits", "id"),),
	)

I'm going to detect if the item_table is missing that _commit column and add it.