fix calReleaseNumCommitsBehind (#24148) (#24197)

Backport #24148

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

_contributed in the context of @forgejo_

---------

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

models/fixtures/repo_unit.yml

@@ -1,3 +1,4 @@
+# See models/unit/unit.go for the meaning of the type
 -
   id: 1
   repo_id: 1
@@ -569,3 +570,29 @@
   type: 3
   config: "{\"IgnoreWhitespaceConflicts\":false,\"AllowMerge\":true,\"AllowRebase\":true,\"AllowRebaseMerge\":true,\"AllowSquash\":true}"
   created_unix: 946684810
+
+-
+  id: 85
+  repo_id: 57
+  type: 1
+  created_unix: 946684810
+-
+  id: 86
+  repo_id: 57
+  type: 2
+  created_unix: 946684810
+-
+  id: 87
+  repo_id: 57
+  type: 3
+  created_unix: 946684810
+-
+  id: 88
+  repo_id: 57
+  type: 4
+  created_unix: 946684810
+-
+  id: 89
+  repo_id: 57
+  type: 5
+  created_unix: 946684810