Fix `View File` button link if branch deleted on pull request files pages (#31063)

as title

routers/web/repo/pull.go

@@ -862,7 +862,7 @@ func viewPullFiles(ctx *context.Context, specifiedStartCommit, specifiedEndCommi
 		}
 
 		if pull.HeadRepo != nil {
-			ctx.Data["SourcePath"] = pull.HeadRepo.Link() + "/src/branch/" + util.PathEscapeSegments(pull.HeadBranch)
+			ctx.Data["SourcePath"] = pull.HeadRepo.Link() + "/src/commit/" + endCommitID
 
 			if !pull.HasMerged && ctx.Doer != nil {
 				perm, err := access_model.GetUserRepoPermission(ctx, pull.HeadRepo, ctx.Doer)