panic in tracing

[extrawurst]

06:45:16 [ERROR] panic: PanicInfo { payload: Any { .. }, message: Some(called `Result::unwrap()` on an `Err` value: Utf8Error { valid_up_to: 17, error_len: Some(1) }), location: Location { file: "/Users/brew/Library/Caches/Homebrew/cargo_cache/registry/src/github.com-1ecc6299db9ec823/git2-0.13.21/src/tracing.rs", line: 79, col: 58 } }
trace:
   0: backtrace::capture::Backtrace::new
   1: gitui::set_panic_handlers::{{closure}}
   2: std::panicking::rust_panic_with_hook
   3: std::panicking::begin_panic_handler::{{closure}}
   4: std::sys_common::backtrace::__rust_end_short_backtrace
   5: _rust_begin_unwind
   6: core::panicking::panic_fmt
   7: core::result::unwrap_failed
   8: core::result::Result<T,E>::unwrap
   9: git2::tracing::tracing_cb_c
  10: _git_trace__write_fmt
  11: _stream_write
  12: _git_http_client_send_body
  13: _http_stream_write
  14: _stream_thunk
  15: _write_one
  16: _write_pack
  17: _git_smart__push
  18: _git_push_finish
  19: _git_remote_upload
  20: _git_remote_push
  21: git2::remote::Remote::push
  22: std::sys_common::backtrace::__rust_begin_short_backtrace
  23: core::ops::function::FnOnce::call_once{{vtable.shim}}
  24: std::sys::unix::thread::Thread::new::thread_start
  25: _pthread_jit_write_protect_np

this was reported here: extrawurst/gitui#881 (comment)

probably caused because our tracing implementation (the callback) did not expect libgit2 sending non-utf8 tracing. turns out libgit2 traces arbitrary byte buffers. so its no wonder our to_str.unwrap chokes on it 🙈