Paths users specify in `ignore` configuraiton option is relative to the
directory which contains the rustfmt.toml file. When processing the ignore paths
internally, rustfmt should add a path to the directory as a prefix since
RealPath passed from libsyntax is a full path.
```
---- ignore_path::test::test_ignore_path_set stdout ----
Warning: can't set `ignore = IgnoreList({"foo.rs", "bar_dir/*"})`, unstable features are only available in nightly channel.
thread 'ignore_path::test::test_ignore_path_set' panicked at 'assertion failed: ignore_path_set.is_match(&FileName::Real(PathBuf::from("src/foo.rs")))', src/ignore_path.rs:51:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
failures:
ignore_path::test::test_ignore_path_set
```
