fix: use fastRelativePath when processing ignore paths

[43081j]

An ignore path can be relative.

For example:

• ignore folder can be /foo/configs
• file can be /foo/src/index.ts
• the ignore file can be /foo/configs/.prettierignore
• the ignore contents can be ../src/index.ts

This means we end up doing:

// does not fall back to path.relative
fastRelativeChildPath('/foo/configs', '/foo/src/index.ts');
// undefined

Which fails since we actually need fastRelativePath:

// falls back to path.relative
fastRelativePath('/foo/configs', '/foo/src/index.ts');
// ../src/index.ts

Fixes #82