Support flags getting modified by hooks at buffer creation

Fixes #361

src/buffer.cc

@@ -55,7 +55,8 @@ Buffer::Buffer(String name, Flags flags, BufferLines lines,
     run_hook_in_own_context("BufCreate", m_name);
 
     // now we may begin to record undo data
-    m_flags = flags;
+    if (not (flags & Flags::NoUndo))
+        m_flags &= ~Flags::NoUndo;
 
     for (auto& option : options().flatten_options())
         on_option_changed(*option);

src/buffer_utils.cc

@@ -119,6 +119,8 @@ Buffer* create_fifo_buffer(String name, int fd, bool scroll)
         if (mode != EventMode::Normal)
             return;
 
+        kak_assert(buffer->flags() & Buffer::Flags::Fifo);
+
         constexpr size_t buffer_size = 2048;
         // if we read data slower than it arrives in the fifo, limiting the
         // iteration number allows us to go back go back to the event loop and