Make jv_sort stable regardless of qsort details.

[stedolan]

No description provided.

[nicowilliams]

Oh! I should have looked closely. I didn't know we had an index to look at. Ha! Good one!

Rebased so it's a fast-forward push and then I'll restore the sort_by tests to tests/jq.test.

[nicowilliams]

Thanks @stedolan!

[pkoppstein]

I didn't know we had an index to look at

Adding the index was part of the update.

The FAQ now records this monumental event.

Thanks y'all.

[nicowilliams]

@pkoppstein Let me rephrase. I hadn't understood that we had struct stort_entry to which an index could be added. I simply hadn't stopped to think about it, or even look.