Columns beginning with an underscore do not facet correctly

[simonw]

Datasette treats columns that start with an underscore as querystring parameters it should ignore!

Discovered in simonw/git-history#14 (comment)

[simonw]

I think the answer is to spot this case and link to ?_item_exact=x instead of ?_item=x - it looks like that's already recommended in the documentation here: https://docs.datasette.io/en/stable/json_api.html#column-filter-arguments

?column__exact=value or ?_column=value
Returns rows where the specified column exactly matches the value.

So maybe the facet selection rendering logic needs to spot this and link correctly to it?