CaiCandong c576b50441
Fix being unable to use a repo that prohibits accepting PRs as a PR source. (#26785)
## Description 
Sometimes, we need to use an upstream mirror repository to update the
current development repository, but mirror repositories are prohibited
from PR. It should not appear in `merge to,` but it can appear in `pull
from.`
Fix #24585 #26193 #26781
Related #24183

Many thanks to @apnote  for assisting me in reproducing this bug!

## ScreenShot
---
### Before

<img
src="https://github.com/go-gitea/gitea/assets/50507092/3d76c376-1f54-45b9-80c9-6ba8319d6a9a"
width="400px">

<img
src="https://github.com/go-gitea/gitea/assets/50507092/fbfd9f7f-421f-4a2e-9a3e-f2958bbf3312"
width="400px">

### After

<img
src="https://github.com/go-gitea/gitea/assets/50507092/e6984524-4f61-4310-b795-4d8598bd8963"
width="400px">

<img
src="https://github.com/go-gitea/gitea/assets/50507092/04065b44-78d7-4721-bf31-0f1674150727"
width="400px">
2023-08-29 12:07:15 +03:00
..
2023-06-15 15:12:08 +00:00
2023-08-22 09:20:09 +00:00
2022-06-27 15:58:46 -05:00
2023-04-29 15:43:01 -04:00