[rfc] flip rerere.enabled default to be "on" at Git 3.0 boundary?

A good default matters, and people who find out how useful a rerere database is would say "gee, that sounds great but why they do not enable it by default? It is too buggy and they wanted to reduce the number of support requests?" Yes, the reason it is not enabled by default initially was exactly that, i.e. those opt into the feature was used as guinea pigs to polish the feature. But we forgot to set the graduation criteria and never said "ok it is mature enough, so let's turn it on for everybody".

Perhaps Git 3.0 boundary is a good occasion to do so?