bug#15101: 24.3.50; debugger-eval-expression broken

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Start Emacs with: emacs -Q --eval '(let ((foo 123)) (debug))'
> Then press e. Enter foo to evaluate the local variable foo. But it
> doesn't work; it only generates the error:
> debugger-eval-expression: Symbol's value as variable is void: foo
> This used to work fine in previous versions.

Indeed, this is a change that will trip up users.  Here's what's
happening: `e' will now run the code in the context in which the "code
on the current line" was run.

This refinement can be useful for dynamically bound variables, but was
mostly added for lexically bound variables, where it's indispensable.

So the above recipe works again if you use C-p before `e' so that point
is now on the top-most line, which stands for "in the context that
called `debug'".

I think a good fix is to change debug.el so that point starts on the
first line of the *Debugger* buffer rather than on the second.


        Stefan