Decrement Fork Num when converting from Fork (#17035)

When converting repositories from forks to normal the root NumFork needs to be
decremented too.

Fix #17026

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

models/repo.go

@@ -1209,6 +1209,12 @@ func IncrementRepoForkNum(ctx DBContext, repoID int64) error {
 	return err
 }
 
+// DecrementRepoForkNum decrement repository fork number
+func DecrementRepoForkNum(ctx DBContext, repoID int64) error {
+	_, err := ctx.e.Exec("UPDATE `repository` SET num_forks=num_forks-1 WHERE id=?", repoID)
+	return err
+}
+
 // ChangeRepositoryName changes all corresponding setting from old repository name to new one.
 func ChangeRepositoryName(doer *User, repo *Repository, newRepoName string) (err error) {
 	oldRepoName := repo.Name