Auto merge of #106415 - Nilstrieb:where-is-my-master-branch, r=jyn514

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

People who do have a remote for `rust-lang/rust` but don't have the master branch checked out there used to get this error when running `x fmt`:
> fatal: ambiguous argument 'rust/master': unknown revision or path not in the working tree.
> Use '--' to separate paths from revisions, like this:
> 'git <command> [<revision>...] -- [<file>...]'
> rust/master

Which is not exactly helpful.

Now, we fall back to `origin/master` (hoping that at least that remote exists) for that case. If there is still some error, we just fall back to `x fmt .` and print a warning.

r? `@jyn514`

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