Get rules by id when editing branch protection rule (#22932)

When users rename an existing branch protection rule, a new rule with
the new name will be created and the old rule will still exist.

![image](https://user-images.githubusercontent.com/15528715/219276442-d3c001ad-e693-44ec-9ad2-b33f2666b49b.png)

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![image](https://user-images.githubusercontent.com/15528715/219276478-547c3b93-b3f1-4292-a1ef-c1b7747fe1bb.png)

The reason is that the `SettingsProtectedBranchPost` function only get
branch protection rule by name before updating or creating a rule. When
the rule name changes, the function cannot find the existing rule so it
will create a new rule rather than update the existing rule. To fix the
bug, the function should get rule by id first.

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Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/forms/repo_form.go

@@ -190,6 +190,7 @@ func (f *RepoSettingForm) Validate(req *http.Request, errs binding.Errors) bindi
 // ProtectBranchForm form for changing protected branch settings
 type ProtectBranchForm struct {
 	RuleName                      string `binding:"Required"`
+	RuleID                        int64
 	EnablePush                    string
 	WhitelistUsers                string
 	WhitelistTeams                string