User would keep seeing an empty repo if:
* An error occurs during the first git pushing/receiving
* A user replaces the Gitea's empty repository manually
Fix: when a user is viewing the repository web page, if the repoModal.IsEmpty is true, we check the git repository again to detect whether it is really empty.
However: the IsEmpty flag is deeply broken and should be removed. For example it's possible for a repository to be non-empty by that flag but still 500 because there are no branches - only tags -or the default branch is non-extant as it has been 0-pushed.
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