repl-interacter: save history after entering every line

Fixes: https://git.lix.systems/lix-project/lix/issues/328
Change-Id: Iedd79ff5f72e84766ebd234c63856170afc624f0

doc/manual/rl-next/repl-fix-history.md

@@ -0,0 +1,9 @@
+---
+synopsis: "`nix repl` history is saved more reliably"
+cls: 1164
+credits: puck
+---
+
+`nix repl` now saves its history file after each line, rather than at the end
+of the session; ensuring that it will remember what you typed even after it
+crashes.

src/libcmd/repl-interacter.cc

@@ -175,6 +175,8 @@ bool ReadlineLikeInteracter::getLine(std::string & input, ReplPromptType promptT
 
     if (!s)
         return false;
+
+    write_history(historyFile.c_str());
     input += s;
     input += '\n';
     return true;