bug#23871: 25.1.50; Undo unexpectedly leads to blank buffer

[Stefan Monnier]

> One issue I noticed with the current undo system is that it behaves
> differently if the command that inserts text is invoked via a keyboard
> shortcut instead of via M-x ... RET.

This is probably because of the

    buffer_before_last_command_or_undo == current_buffer

test, which is a naive/conservative test that just punts if there's
a buffer switch (in which case point_before_last_command_or_undo is
simply meaningless).  And in this case there is, since the last command
was in another buffer (the last command was the RET you executed in the
minibuffer).

We could probably make it work by saving&restoring
buffer_before_last_command_or_undo and point_before_last_command_or_undo
around the minibuffer thingy.  Or, making
point_before_last_command_or_undo into a buffer-local variable and get
rid of buffer_before_last_command_or_undo.


        Stefan