NixRepl::mainLoop: restore old curRepl on function exit

This fixes completion callbacks after entering and leaving a nested
debugger.

src/libcmd/repl.cc

@@ -252,7 +252,9 @@ void NixRepl::mainLoop()
     el_hist_size = 1000;
 #endif
     read_history(historyFile.c_str());
+    auto oldRepl = curRepl;
     curRepl = this;
+    Finally restoreRepl([&] { curRepl = oldRepl; });
 #ifndef READLINE
     rl_set_complete_func(completionCallback);
     rl_set_list_possib_func(listPossibleCallback);