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/integration/api_repo_test.go

@@ -82,9 +82,9 @@ func TestAPISearchRepo(t *testing.T) {
 	}{
 		{
 			name: "RepositoriesMax50", requestURL: "/api/v1/repos/search?limit=50&private=false", expectedResults: expectedResults{
-				nil:   {count: 31},
-				user:  {count: 31},
-				user2: {count: 31},
+				nil:   {count: 32},
+				user:  {count: 32},
+				user2: {count: 32},
 			},
 		},
 		{