Add support for forking single branch (#25821)

Fixes #25117

Add UI for choosing branch to fork

Change default branch on single-branch forks


![image](https://github.com/go-gitea/gitea/assets/19504461/28505f69-a9a2-43a8-8b19-a0cdac3ddc5a)

---------

Co-authored-by: Denys Konovalov <kontakt@denyskon.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/forms/repo_form.go

@@ -51,6 +51,8 @@ type CreateRepoForm struct {
 	Labels          bool
 	ProtectedBranch bool
 	TrustModel      string
+
+	ForkSingleBranch string
 }
 
 // Validate validates the fields