Refresh the refernce of the closed PR when reopening (#24231)

Close #24213 
Replace #23830

#### Cause

- Before, in order to making PR can get latest commit after reopening,
the `ref`(${REPO_PATH}/refs/pull/${PR_INDEX}/head) of evrey closed PR
will be updated when pushing commits to the `head branch` of the closed
PR.

#### Changes

- For closed PR , won't perform these behavior: insert`comment`, push
`notification` (UI and email), exectue
[pushToBaseRepo](https://github.com/go-gitea/gitea/blob/74225033413dc0f2b308bbe069f6d185b551e364/services/pull/pull.go#L409)
function and trigger `action` any more when pushing to the `head branch`
of the closed PR.
- Refresh the reference of the PR when reopening the closed PR (**even
if the head branch has been deleted before**). Make the reference of PR
consistent with the `head branch`.

models/issues/pull.go

@@ -433,6 +433,10 @@ func (pr *PullRequest) GetGitRefName() string {
 	return fmt.Sprintf("%s%d/head", git.PullPrefix, pr.Index)
 }
 
+func (pr *PullRequest) GetGitHeadBranchRefName() string {
+	return fmt.Sprintf("%s%s", git.BranchPrefix, pr.HeadBranch)
+}
+
 // IsChecking returns true if this pull request is still checking conflict.
 func (pr *PullRequest) IsChecking() bool {
 	return pr.Status == PullRequestStatusChecking