Enforce setting HEAD in wiki to master (#13950 An error occurred )

The default branch in wikis must be master - therefore forcibly set the HEAD
to master.

Fix #13846
  
An error occurred
 


Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>

services/wiki/wiki.go

@@ -77,6 +77,8 @@ func InitWiki(repo *models.Repository) error {
 		return fmt.Errorf("InitRepository: %v", err)
 	} else if err = repo_module.CreateDelegateHooks(repo.WikiPath()); err != nil {
 		return fmt.Errorf("createDelegateHooks: %v", err)
+	} else if _, err = git.NewCommand("symbolic-ref", "HEAD", git.BranchPrefix+"master").RunInDir(repo.WikiPath()); err != nil {
+		return fmt.Errorf("unable to set default wiki branch to master: %v", err)
 	}
 	return nil
 }