fix reopen logic for agit flow pull request (#26399)

the head branch is meaningless for a agit flow pull request, so should
not check it when reopen it.

related: #24231
fix #26334

Signed-off-by: a1012112796 <1012112796@qq.com>

routers/web/repo/issue.go

@@ -2935,6 +2935,7 @@ func NewComment(ctx *context.Context) {
 
 				// check whether the ref of PR <refs/pulls/pr_index/head> in base repo is consistent with the head commit of head branch in the head repo
 				// get head commit of PR
+				if pull.Flow == issues_model.PullRequestFlowGithub {
 					prHeadRef := pull.GetGitRefName()
 					if err := pull.LoadBaseRepo(ctx); err != nil {
 						ctx.ServerError("Unable to load base repo", err)
@@ -2983,6 +2984,7 @@ func NewComment(ctx *context.Context) {
 						}
 					}
 				}
+			}
 
 			if pr != nil {
 				ctx.Flash.Info(ctx.Tr("repo.pulls.open_unmerged_pull_exists", pr.Index))