housekeeping: Enable writing MIDX by default
We have recently introduced the logic to write multi-pack indices during repository maintenance. This new data structure allows Git to look up objects more efficiently when there are many packfiles in a repository by providing an index that spans across all the packfiles.
The code has been rolled out to production systems without any observed issues. As this is a significant new data structure though we're playing it safe and release the feature with its flag default enabled.
Changelog: changed
Part of #4741 (closed).