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/info/exclude

@@ -0,0 +1,6 @@
+# git ls-files --others --exclude-from=.git/info/exclude
+# Lines that start with '#' are comments.
+# For a project mostly in C, the following would be a good set of
+# exclude patterns (uncomment them if you want to use them):
+# *.[oa]
+# *~