This is a request for a new filter in Package Menu.

Motivation: The command `package-menu-mark-upgrades' in the "*Packages*"
buffer marks all packages that have new versions. Both the new versions
are marked for installation (I) and the old versions for deletion (D).

However, it is not so easy to inspect which packages will be
affected. Currently the only simple ways are to do an `isearch' in the
"*Packages*" buffer for "^[^ ]", or to do an `occur' for the same
regexp. The latter gives an overview of all packages involved, but
requires an indirection step to get into the "*Packages*" buffer, for
example to unmark a package.

The attached file adds a new command to filter the "*Packages*"
buffer to show only the packages with a non-empty mark, i.e. the same as
the above `occur' command would show, but then without the extra
indirection.

My proposal is to bind it to "/m", and to add a menu entry.

This function is dependent on commit aea12d4903 (Bug#38424), which
introduces other filters. Contrary to the other filters, this filter
keeps the marks intact. Otherwise it would not be useful.
(Other filters clear the marks, because the marks are not part of the
data structure that builds the package list, but only of the
representation in the buffer.)

If this filter is accepted, I will make it into a full-blown patch with
documentation, etc.) The current format is fit to be included in the `init' file.