`x fmt` tries to format non-rust files

[Noratrieb]

formatting modified file compiler/rustc_error_messages/locales/en-US/borrowck.ftl
error: unknown start of token: `
 --> /home/nilsh/projects/rust/compiler/rustc_error_messages/locales/en-US/borrowck.ftl:2:33
  |
2 |     cannot move a value of type `{$ty}`
  |                                 ^
  |
help: Unicode character '`' (Grave Accent) looks like ''' (Single Quote), but it is not
  |
2 |     cannot move a value of type '{$ty}`
  |                                 ~

error: unknown start of token: `

When there was a change in a .ftl or other non-rust file, bootstrap tries to format it, which makes rustfmt very sad.

cc @albertlarsan68 and @jyn514 who broke this in #105702

[jyn514]

I think we just need to filter .rs files before passing them to rustfmt.

[Noratrieb]

This will probably be hit a lot by contributors so I'll pin it for better visibility.

[chenyukang]

@rustbot claim

[albertlarsan68]

Normally, the walker already filters for .rs files, per https://docs.rs/ignore/latest/ignore/struct.WalkBuilder.html. Maybe the ignore crate docs are not fully up to date, or I didn't understand them correctly.

[albertlarsan68]

The workaround is to run x fmt . (notice the dot at the end of the command).

[chenyukang]

I think if we add it like this:

ignore_fmt.add(&format!("!/{}", untracked_path)).expect(&untracked_path);

ignore will always not ignore it anymore.