fix calReleaseNumCommitsBehind (#24148) (#24197)

Backport #24148

`repoCtx.CommitsCount` is not reliably the commit count of the default
branch (Repository.GetCommitsCount depends on what is currently
displayed).

_contributed in the context of @forgejo_

---------

Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

tests/gitea-repositories-meta/user2/repo-release.git/objects/f3/f1c90ac949aa1b0f129d30f338d408663c8a83

@@ -0,0 +1,2 @@
+x
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