[GR-18339] Fix multiline input in repl

Author: timfel

graalpython/com.oracle.graal.python.shell/src/com/oracle/graal/python/shell/GraalPythonMain.java

@@ -642,13 +642,17 @@ public int readEvalPrint(Context context, ConsoleHandler consoleHandler) {
                                 continuePrompt = doEcho ? sys.getMember("ps2").asString() : null;
                             }
                             if (e.isIncompleteSource()) {
-                                // read another line of input
+                                // read more input until we get an empty line
                                 consoleHandler.setPrompt(continuePrompt);
                                 String additionalInput = consoleHandler.readLine();
+                                while (additionalInput != null && !additionalInput.isEmpty()) {
+                                    sb.append(additionalInput).append('\n');
+                                    consoleHandler.setPrompt(continuePrompt);
+                                    additionalInput = consoleHandler.readLine();
+                                }
                                 if (additionalInput == null) {
                                     throw new EOFException();
                                 }
-                                sb.append(additionalInput).append('\n');
                                 // The only continuation in the while loop
                                 continue;
                             } else if (e.isExit()) {