Stable branch quick fix for pager change-mailbox push/exec bug.
The menu functions mutt_push/pop_current_menu() keep track of the menu
stack, automatically setting CurrentMenu when exiting menus.
The only gotcha was the function sharing between the index and pager
menus. The index uses a hack, setting menu->menu to MENU_PAGER for
operations redirecting through the index and back to the pager
afterwards.
I thought this was covered by the restoration of the menu before
returning to the pager, or when exiting the index switch. However it
is not: invoking other menus, such as the browser, will result in
CurrentMenu being set to the Pager when exiting those (by
mutt_pop_current_menu()). This can result in folder hooks failing for
unshared functions.
A better fix is to remove the hack of using menu->menu, because this
can easily cause a problem in the future in other situations. (I will
make this fix in master next.) For the stable branch, I am explicitly
setting/restoring CurrentMenu before invoking the folder hooks.