`git gc` cron should not change repo's `updated_unix` timestamp

[silverwind]

Since #15177, a repo's updated_unix timestamp seen on a repo on a org page can change when the git gc cron runs which I think is undesirable because it makes it seem as thought someone pushed to the repo while actually nothing happend. It executes with this SQL:

models/repo.go:744:updateSize() [I] [SQL] SELECT COALESCE(sum(`size`),0) FROM `lfs_meta_object` WHERE (repository_id = ?) [260] - 750.016µs
.../repository/check.go:96:func1() [I] [SQL] UPDATE `repository` SET `size` = ?, `updated_unix` = ? WHERE `id`=? [41395 1619453525 260] - 2.02599ms

I think updated_unix should not be updated during the git gc cron and instead only update when actual changes happen in the repo files, e.g. push actions.

[lunny]

I think v1.14 should also have the problem.