bug#13492: 24.2.92; Buffer changing in dired-mark-pop-up

[Leo Liu]

In discussion of http://debbugs.gnu.org/13152, it is noted that
dired-mark-pop-up invokes FUNCTION in different buffers depending on if
there are marked files. This is problematic and makes it harder to use
dired-mark-pop-up correctly.

In particular this is directly related to dired-guess-shell-alist-user,
a user variable. Harder to predict the behaviour of COMMAND when it is a
lisp expression. But in this case an error is thrown when the buffer is
switched to a non-dired buffer. So no user code will be broken if
dried-mark-pop-up stops switching buffers.

I propose fix dired-mark-pop-up to be consistent where it invokes
FUNCTION.

Leo