Fix Issue #210: Changes to be undone outside visible portion of buffer

[cjennings]

This PR addresses issue 210:

ROOT CAUSE

On line 1556, in org-msg-edit-mode reads:

org-msg/org-msg.el

Line 1556 in 59e2042

(add-hook 'message-sent-hook 'undo t t)

When sending messages that extend past the visible space, org-msg prepares the content and may narrow the buffer. When the message-sent-hook fires in a narrowed buffer, it calls undo which errors when attempting to undo changes recorded outside the visible restriction. "primitive-undo: Changes to be undone are outside visible portion of buffer". While the email is sent successfully, the compose buffer is left in a mixed text+html state, which causes confusion and forces the users to close buffers manually.

FIX DESCRIPTION

• Replace the raw undo hook with a helper which widens before performing the undo. Add that helper to message-send-hook based on the value of message-kill-buffer-on-exit. This way:
• If message-kill-buffer-on-exit is t, there’s no need to restore the buffer as it will be killed right after the hooks. No undo → no error → no need to widen.
• If the buffer won’t be killed, we DO need to restore it, therefore we widen first and then perform the undo.

ALTERNATIVES REJECTED

• Removing undo entirely. This fixes the error, but was rejected as it changes org-msg's behavior for users who keep the buffer open post-send.
• Saving/restoring the original buffer text explicitly. Rejected because this felt more invasive and might risk clobbering other transient state.
• Always add the hook to widen before performing the undo. Rejected to avoid executing more code than necessary.