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>

models/fixtures/repository.yml

@@ -1,3 +1,4 @@
+# don't forget to add fixtures in repo_unit.yml
 -
   id: 1
   owner_id: 2
@@ -1647,3 +1648,16 @@
   is_private: true
   status: 0
   num_issues: 0
+
+-
+  id: 57
+  owner_id: 2
+  owner_name: user2
+  lower_name: repo-release
+  name: repo-release
+  default_branch: main
+  is_empty: false
+  is_archived: false
+  is_private: false
+  status: 0
+  num_issues: 0