Obsolete messages and previous msgids don't combine well

Matthew East run into this problem with ubuntu-docs/gnome2-user-guide/es/es.po — previous msgids are not correctly "obsoleted", while they need to be for msgfmt -cv checks to pass. I.e. we export a message like:

#| msgid ""
#| "<application>Window Selector</application> lists all your open windows. "
#| "To give focus to a window, click on the window selector icon at the "
#| "extreme right of the top edge panel, then select the window. For more on "
#| "this, see <xref linkend=\"windows-focus\"/>."
#~ msgid ""
#~ "The <application>Window Selector</application> lists all of your open "
#~ "windows. To give focus to a window, click on the window selector icon at the "
#~ "extreme right of the top edge panel, then select the window. For more on "
#~ "this, see <xref linkend=\"windows-focus\"/>."
#~ msgstr ""
#~ "El <application>Selector de ventanas</application> lista todas sus ventanas "
#~ "abiertas. Para darle el foco a una ventana, pulse en el icono del selector "
#~ "de ventanas en el extremo derecho del panel del borde superior, después "
#~ "seleccione la ventana. Para obtener más información acerca de ésto consulte "
#~ "la <xref linkend=\"windows-focus\"/>."

while it should be "#~|" instead of just "#|".

This is easy to fix, but I'd still like us to just remove all "#|" comments.