Fix possible pull request broken when leave the page immediately after clicking the update button (#34509) (#34607)

Backport #34509 by @lunny

If user leaves the page, the context will become cancelled, so that the
update process maybe terminal in an unexpected status. This PR haven't
resolve the problem totally. It uses a background context to not cancel
the update process even if the user leaved the pull request view page.

Fix #31779

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

routers/api/v1/repo/pull.go

@@ -23,6 +23,7 @@ import (
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/gitrepo"
+	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	api "code.gitea.io/gitea/modules/structs"
@@ -1301,7 +1302,7 @@ func UpdatePullRequest(ctx *context.APIContext) {
 	// default merge commit message
 	message := fmt.Sprintf("Merge branch '%s' into %s", pr.BaseBranch, pr.HeadBranch)
 
-	if err = pull_service.Update(ctx, pr, ctx.Doer, message, rebase); err != nil {
+	if err = pull_service.Update(graceful.GetManager().ShutdownContext(), pr, ctx.Doer, message, rebase); err != nil {
 		if pull_service.IsErrMergeConflicts(err) {
 			ctx.APIError(http.StatusConflict, "merge failed because of conflict")
 			return