Improve handling of /dev/disk/by-id/ changes

Preserve previous answer to grub-pc/install_devices if we have to ask
grub-pc/install_devices_disks_changed and the user chooses not to
install to any devices, so that we can recover from temporary bugs that
cause /dev/disk/by-id/ paths to change.

Ideally this would also include explanatory text in the debconf
template, but it's a bit late in the buster release cycle for new
translatable text.  I've left an XXX comment for the time being.

Closes: #919029

debian/changelog

@@ -4,6 +4,10 @@ grub2 (2.02+dfsg1-12) UNRELEASED; urgency=medium
     names under /dev/disk/by-id/.  This migration happened in
     1.98+20100702-1, which was in squeeze (four stable releases ago), so we
     no longer need to carry around this complex code.
+  * Preserve previous answer to grub-pc/install_devices if we have to ask
+    grub-pc/install_devices_disks_changed and the user chooses not to
+    install to any devices, so that we can recover from temporary bugs that
+    cause /dev/disk/by-id/ paths to change (closes: #919029).
 
  -- Colin Watson <cjwatson@debian.org>  Tue, 26 Feb 2019 09:51:36 +0000
 

debian/postinst.in

@@ -578,7 +578,14 @@ case "$1" in
               fi
             done
 
-            if [ "$question" != grub-pc/install_devices ]; then
+            if [ "$question" != grub-pc/install_devices ] && [ "$RET" ]; then
+              # XXX cjwatson 2019-02-26: The description of
+              # grub-pc/install_devices_disks_changed ought to explain that
+              # selecting no devices will leave the configuration unchanged
+              # so that you'll be prompted again next time, but it's a bit
+              # close to the Debian 10 release to be introducing new
+              # translatable text.  For now, it should be sufficient to
+              # avoid losing configuration data.
               db_set grub-pc/install_devices "$RET"
               db_fset grub-pc/install_devices seen true
             fi
@@ -619,7 +626,7 @@ case "$1" in
               fi
             fi
 
-            db_get grub-pc/install_devices
+            db_get "$question"
             if [ -z "$RET" ]; then
               # Reset the seen flag if the current answer is false, since
               # otherwise we'll loop with no indication of why.