fix calReleaseNumCommitsBehind (#24148)

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

For instance on the releases page the commit count is correct:
https://codeberg.org/Codeberg/pages-server/releases


![2023-04-15-215027](https://user-images.githubusercontent.com/3864879/232250500-6c05dc00-7030-4ec9-87f1-18c7797d36bf.png)

However it is not on the single page:
https://codeberg.org/Codeberg/pages-server/releases/tag/v4.6.2


![2023-04-15-215036](https://user-images.githubusercontent.com/3864879/232250503-620c8038-7c2c-45a1-b99d-cb994ef955a6.png)

This PR fixes this by removing a "fast branch" which was using this
field (I think this field should be removed, since it is a bit
unpredictable - but this would mean a larger refactoring PR).

_contributed in the context of @forgejo_

---------

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

tests/gitea-repositories-meta/user2/repo-release.git/hooks/pre-merge-commit.sample

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# An example hook script to verify what is about to be committed.
+# Called by "git merge" with no arguments.  The hook should
+# exit with non-zero status after issuing an appropriate message to
+# stderr if it wants to stop the merge commit.
+#
+# To enable this hook, rename this file to "pre-merge-commit".
+
+. git-sh-setup
+test -x "$GIT_DIR/hooks/pre-commit" &&
+        exec "$GIT_DIR/hooks/pre-commit"
+: