Make sure to reset commit count in the cache on mirror syncing (#4720)

* Make sure to reset commit count in the cache on mirror syncing

* reset count of commits in all branches

models/repo_mirror.go

 	"time"
 
 	"code.gitea.io/git"
+	"code.gitea.io/gitea/modules/cache"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/process"
 	"code.gitea.io/gitea/modules/setting"
 		}
 	}
 
+	branches, err := m.Repo.GetBranches()
+	if err != nil {
+		log.Error(4, "GetBranches: %v", err)
+		return false
+	}
+
+	for i := range branches {
+		cache.Remove(m.Repo.GetCommitsCountCacheKey(branches[i].Name, true))
+	}
+
 	m.UpdatedUnix = util.TimeStampNow()
 	return true
 }