panic, when selecting `mark baseline` two times

[hellow554]

# cargo new foo
# cd foo/src/
# env RUST_BACKTRACE=1 cargo-bisect-rustc --test-dir .

selecting mark baseline in both cases

checking nightly-2018-05-25
std for x86_64-unknown-linux-gnu: 51.11 MB / 51.11 MB [===========================================================================================================] 100.00 % 2.73 MB/s      Finished dev [unoptimized + debuginfo] target(s) in 0.03s
uninstalling nightly-2018-05-25


nightly-2018-05-25 finished with exit code Some(0).
please select an action to take:
verifying the start of the range does not reproduce the regression
std for x86_64-unknown-linux-gnu: 51.11 MB / 51.11 MB [===========================================================================================================] 100.00 % 2.72 MB/s      Finished dev [unoptimized + debuginfo] target(s) in 0.00s
uninstalling nightly-2018-05-25


nightly-2018-05-25 finished with exit code Some(0).
please select an action to take:
uninstalling nightly-2018-05-25
tested nightly-2018-05-25, got No
confirmed the start of the range does not reproduce the regression
verifying the end of the range reproduces the regression
thread 'main' panicked at 'the end of the range to test must reproduce the regression', src/least_satisfying.rs:34:14
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:463
   5: std::panicking::begin_panic
   6: cargo_bisect_rustc::least_satisfying::least_satisfying
   7: cargo_bisect_rustc::run
   8: cargo_bisect_rustc::main
   9: std::rt::lang_start::{{closure}}
  10: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  11: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  12: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:374
             at libstd/rt.rs:58
  13: main
  14: __libc_start_main
  15: _start