crash: #[path="/dev/null"] attr

This code causes rustfmt to crash

#[path="/dev/null"]
pub mod null;

thread 'main' panicked at 'path is expected to be under the root', /cargo/registry/src/github.com-1ecc6299db9ec823/ignore-0.4.16/src/gitignore.rs:227:9
stack backtrace:
   0:     0x5587c3e2eaa7 - std::backtrace_rs::backtrace::libunwind::trace::h746c3e9529d524bc
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/../../backtrace/src/backtrace/libunwind.rs:90:5
   1:     0x5587c3e2eaa7 - std::backtrace_rs::backtrace::trace_unsynchronized::h86340908ff889faa
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x5587c3e2eaa7 - std::sys_common::backtrace::_print_fmt::h43f85f9b18230404
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x5587c3e2eaa7 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hc132ae1a5b5aa7cd
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x5587c3e5283c - core::fmt::write::hdf023a0036d2a25f
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/core/src/fmt/mod.rs:1078:17
   5:     0x5587c3e2b462 - std::io::Write::write_fmt::h8580846154bcb66a
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/io/mod.rs:1519:15
   6:     0x5587c3e30a75 - std::sys_common::backtrace::_print::h7ee55fed88d107a3
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x5587c3e30a75 - std::sys_common::backtrace::print::h54a7d3e52a524177
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x5587c3e30a75 - std::panicking::default_hook::{{closure}}::h60921e857bf55a40
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/panicking.rs:208:50
   9:     0x5587c3e305ca - std::panicking::default_hook::hf0f9afb1017317fc
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/panicking.rs:225:9
  10:     0x5587c3e31211 - std::panicking::rust_panic_with_hook::h8d66bf42b407aaea
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/panicking.rs:591:17
  11:     0x5587c3ae67c4 - std::panicking::begin_panic::{{closure}}::hbe262b95ef2ff8f9
  12:     0x5587c3ae66bc - std::sys_common::backtrace::__rust_end_short_backtrace::hd70d74b6b6dc38ff
  13:     0x5587c3a2ddac - std::panicking::begin_panic::h998094cd3d11af60
  14:     0x5587c3a5e258 - ignore::gitignore::Gitignore::matched_path_or_any_parents::h687892c59c3a1cae
  15:     0x5587c3b66149 - rustfmt_nightly::syntux::session::ParseSess::ignore_file::h60741d92df2c771d
  16:     0x5587c39bfae0 - rustfmt_nightly::formatting::format_project::h75f3cb078ea2edba
  17:     0x5587c39c93ee - scoped_tls::ScopedKey<T>::set::h46570b94864b4223
  18:     0x5587c39c3789 - rustfmt_nightly::Session<T>::format::h16e5c985cca65c6b
  19:     0x5587c39d38fb - rustfmt::format_and_emit_report::h34b4f1b50145fd6f
  20:     0x5587c39c309d - rustfmt_nightly::Session<T>::override_config::h588d4c0abb82dcad
  21:     0x5587c39d0f8d - rustfmt::execute::hbead43f61fce9ab6
  22:     0x5587c39cd9fd - rustfmt::main::hdc579be80dab480f
  23:     0x5587c39c58b3 - std::sys_common::backtrace::__rust_begin_short_backtrace::h425146d795f70aec
  24:     0x5587c39c5c09 - std::rt::lang_start::{{closure}}::h1a880688683e8956
  25:     0x5587c3e31727 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h54c39b9b8451875a
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/core/src/ops/function.rs:259:13
  26:     0x5587c3e31727 - std::panicking::try::do_call::hcdfba51a06e21b42
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/panicking.rs:379:40
  27:     0x5587c3e31727 - std::panicking::try::hb36953ccd24f930d
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/panicking.rs:343:19
  28:     0x5587c3e31727 - std::panic::catch_unwind::hbb374cd03beb3824
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/panic.rs:396:14
  29:     0x5587c3e31727 - std::rt::lang_start_internal::h97dca9624db2aa85
                               at /rustc/2225ee1b62ff089917434aefd9b2bf509cfa087f/library/std/src/rt.rs:51:25
  30:     0x5587c39d4552 - main
  31:     0x7f0254b48152 - __libc_start_main
  32:     0x5587c39b9f59 - <unknown>

rustfmt 1.4.29-nightly (70ce182 2020-12-04)

Reproduce the crash with the #[path="/dev/null"] module shown in the issue, then trace rustfmt_nightly::syntux::session::ParseSess::ignore_file and rustfmt_nightly::formatting::format_project from the stack trace. Done means rustfmt handles this input without panicking.