Fixed ICE when formatting files outside of toml dir

[aznszn]

Fixes: #6843

The panic was happening due to a line in the ignore crate:

assert!(!path.has_root(), "path is expected to be under the root");

Fix

• Added a guard that returns false when checking if a file matches an ignore pattern, if the ignore rule's directory is not an ancestor of the file's absolute path.
• Added a regression test

N.b.: This assumes that rustfmt ignore rules are supposed to be for that directory and below

[aznszn]

i will fix the CI soon

[aznszn]

@rustbot ready

[matthewhughes934]

If the underlying bug is: someone tried to invoke rustfmt with a --config-path that is outside the tree, then

• Before: we panic
• Now: we silently ignore it, since is_match always returns false here

I'm wondering if instead we could do one of:

• Print an error to the user, telling them that the config path isn't valid
• Try to make sense of it: is there anything we could reasonably do here? If we're running in cargo fmt could we instead use the path to Cargo.toml as the root of ignores?

It doesn't help that --config-path is a bit confusing #5206