Handle non-existent upstream master branches in `x fmt`#106415

Merged

bors merged 2 commits intorust-lang:masterfrom

Noratrieb:where-is-my-master-branch

Jan 7, 2023

Commits on Jan 3, 2023

Add `build_helper` crate to share code between tidy and bootstrap
Noratrieb
committed

Commits on Jan 6, 2023

Handle non-existant upstream master branches in `x fmt`
Noratrieb
committed