Add cache for branch divergence on branch list page (#29577)

The branch page for blender project will take 6s because calculating
divergence is very slow.
This PR will add a cache for the branch divergence calculation. So when
the second visit the branch list, it will take only less 200ms.

services/repository/push.go

@@ -220,6 +220,11 @@ func pushUpdates(optsList []*repo_module.PushUpdateOptions) error {
 					}
 				}
 
+				// delete cache for divergence
+				if err := DelDivergenceFromCache(repo.ID, branch); err != nil {
+					log.Error("DelDivergenceFromCache: %v", err)
+				}
+
 				commits := repo_module.GitToPushCommits(l)
 				commits.HeadCommit = repo_module.CommitToPushCommit(newCommit)