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/repo/repo_list_test.go

@@ -235,12 +235,12 @@ func TestSearchRepository(t *testing.T) {
 		{
 			name:  "AllPublic/PublicRepositoriesOfUserIncludingCollaborative",
 			opts:  &repo_model.SearchRepoOptions{ListOptions: db.ListOptions{Page: 1, PageSize: 10}, OwnerID: 15, AllPublic: true, Template: util.OptionalBoolFalse},
-			count: 29,
+			count: 30,
 		},
 		{
 			name:  "AllPublic/PublicAndPrivateRepositoriesOfUserIncludingCollaborative",
 			opts:  &repo_model.SearchRepoOptions{ListOptions: db.ListOptions{Page: 1, PageSize: 10}, OwnerID: 15, Private: true, AllPublic: true, AllLimited: true, Template: util.OptionalBoolFalse},
-			count: 34,
+			count: 35,
 		},
 		{
 			name:  "AllPublic/PublicAndPrivateRepositoriesOfUserIncludingCollaborativeByName",
@@ -255,7 +255,7 @@ func TestSearchRepository(t *testing.T) {
 		{
 			name:  "AllPublic/PublicRepositoriesOfOrganization",
 			opts:  &repo_model.SearchRepoOptions{ListOptions: db.ListOptions{Page: 1, PageSize: 10}, OwnerID: 17, AllPublic: true, Collaborate: util.OptionalBoolFalse, Template: util.OptionalBoolFalse},
-			count: 29,
+			count: 30,
 		},
 		{
 			name:  "AllTemplates",