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
<01><>A
+<EFBFBD> <05><><14><0B>o<EFBFBD><6F>RJ<52><4A><EFBFBD><EFBFBD>i 6<><18><><EFBFBD>+t;0<><30><EFBFBD>ֹkK<6B><4B><1B><>	1<><31>4<EFBFBD><34>d<EFBFBD><64>5$ao<>g;<3B><>>]St6F<36><46>ERNț4MgeHd<48>0'V|<7C><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>~p)