Improve speed to huge mono repo with different target branch

[yuyuvn]

Is your feature request related to a problem? Please describe.
We using a huge monorepo with multi argocd application.
Each application has a different git write back branch.
There are case that 1 image change create more than 300 application update. Each update need to wait for each other because we have a mutex for same git repo url.

Describe the solution you'd like
From #177 , we have a mutex when pushing to git. But the key is base on git url. I think we can change mutex key to something like gitUrl:targetBranch

func commitChangesLocked(app *v1alpha1.Application, wbc *WriteBackConfig, state *SyncIterationState, changeList []ChangeEntry) error {
	if wbc.RequiresLocking() {
-		lock := state.GetRepositoryLock(wbc.GitRepo)
+               lock := state.GetRepositoryLock(fmt.Sprintf("%s:%s", wbc.GitRepo, wbc.GitBranch))
		lock.Lock()
		defer lock.Unlock()
	}

	return commitChanges(app, wbc, changeList)
}

Additional context
We don't directly push to master branch because of our company audit policy, which need approve from CODEOWNER to push any change into production.