cli: don't require --allow-backwards when creating a branch

If a branch doesn't exist yet, it's clearly safe to move it "forward"
to any commit (it was simply a bug that we required
`--allow-backards`).

src/commands.rs

@@ -2056,16 +2056,14 @@ fn cmd_backout(
 
 fn is_fast_forward(repo: RepoRef, branch_name: &str, new_target_id: &CommitId) -> bool {
     if let Some(current_target) = repo.view().get_local_branch(branch_name) {
-        if current_target
+        current_target
             .adds()
             .iter()
             .any(|add| repo.index().is_ancestor(add, new_target_id))
-        {
-            return true;
+    } else {
+        true
     }
 }
-    false
-}
 
 fn cmd_branch(
     ui: &mut Ui,