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Fixes#21308.
With the original frontend template
`templates/repo/settings/options.tmpl`, the field
`push_mirror_sync_on_commit` is always empty even when checkbox is
checked. Removing `value` from the input tag seems to solve the issue,
and will set `push_mirror_sync_on_commit: on` when the checkbox is
checked.
(I'm not familiar with the frontend logics Gitea is using, so I don't
really understand the cause of it)
Chongyi Zheng