better fix for commit 3ffc2ed8f

I'm giving up finding a more generalized solution. The issue is that we
need the correct selection state right up to the point where we modify
Lines, in order to capture precise undo state.

Hopefully there aren't any other keychords that should clear the
selection.

text.lua

@@ -172,6 +172,7 @@ function Text.keychord_pressed(chord)
     local before_line = Cursor1.line
     local before = snapshot(before_line)
     Text.insert_return()
+    Selection1 = {}
     if (Cursor_y + Line_height) > App.screen.height then
       Text.snap_cursor_to_bottom_of_screen()
     end