gc: add `--expire-to` option
This commit extends the functionality of `git gc` by adding a new option, `--expire-to=<dir>`. Previously, this feature was implemented in 91badeba32 (builtin/repack.c: implement `--expire-to` for storing pruned objects, 2022-10-24), which allowing users to specify a directory where unreachable and expired cruft packs are stored during garbage collection. However, users had to run `git repack --cruft --expire-to=<dir>` followed by `git prune` to achieve similar results within `git gc`. By introducing `--expire-to=<dir>` directly into `git gc`, we simplify the process for users who wish to manage their repository's cleanup more efficiently. This change involves passing the `--expire-to=<dir>` parameter through to `git repack`, making it easier for users to set up a backup location for cruft packs that will be pruned. Due to the original `git gc --prune=now` deleting all unreachable objects by passing the `-a` parameter to git repack. With the addition of the `--cruft` and `--expire-to` options, it is necessary to modify this default behavior: instead of deleting these unreachable objects, they should be merged into a cruft pack and collected in a specified directory. Therefore, we do not pass `-a` to the repack command but instead pass `--cruft`, `--expire-to`, and `--cruft-expiration=now` to repack. Signed-off-by: ZheNing Hu <adlternative@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>