Crash during rebase

[purcell]

Describe the bug

Got a panic from lorri, apparently as a result of a chain of modifications within a single project in the course of a git rebase.

To Reproduce

Unsure how to reliably reproduce, but would guess at rebasing inside a project which takes a short while to rebuild / re-evaluate.

Expected behavior

I'd hope that lorri itself would not crash, while the underlying nix rebuild might fail if files are appearing/disappearing.

Metadata

$ lorri info --shell-file shell.nix
lorri version: 1.0
gc roots exist, shell_gc_root: "/home/steve/.cache/lorri/gc_roots/85034d9c78b6dd30e9ed4ebdf475ed72/gc_root/shell_gc_root"

$ uname -a
Linux xxxxxxxxxx 4.19.118 #1-NixOS SMP Thu Apr 23 08:30:24 UTC 2020 x86_64 GNU/Linux

Additional context

Crash report:

name = 'lorri'
operating_system = 'unix:Unknown'
crate_version = '1.0.0'
explanation = '''
Panic occurred in file 'src/build_loop.rs' at line 135
'''
cause = '''
Unrecoverable error:
Io {
    msg: "entity not found",
}'''
method = 'Panic'
backtrace = '''

   0: 0x55eed574ec46 - std::panicking::try::do_call::h9d1b51a0e6082453
   1: 0x55eed59853fa - __rust_maybe_catch_panic
   2: 0x55eed57b8816 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h55bb3f53afad9dd6
   3: 0x55eed59791ef - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::he9dcb7073be13c65
   4: 0x55eed5966e30 - std::sys_common::thread::start_thread::h9ce8f903e84fe8e9
   5: 0x55eed5978866 - std::sys::unix::thread::Thread::new::thread_start::h5e894f0575174d9e
   6: 0x7fc52bcdbedd - start_thread
   7: 0x7fc52bbf2a4f - __clone
   8:        0x0 - <unknown>'''