terminal emulator: clear scrollback buffer when resetting to initial state

Fixes utility 'reset' not being able to clear scrollback buffer.

terminal-emulator/src/main/java/com/termux/terminal/TerminalEmulator.java

@@ -1267,6 +1267,7 @@ public final class TerminalEmulator {
                 break;
             case 'c': // RIS - Reset to Initial State (http://vt100.net/docs/vt510-rm/RIS).
                 reset();
+                mMainBuffer.clearTranscript();
                 blockClear(0, 0, mColumns, mRows);
                 setCursorPosition(0, 0);
                 break;