[grubcfg] If we only see #GRUB_DISTRIBUTION, it's not been set

- If we update the line, then GRUB_DISTRIBUTION has been set
 - If we don't update the line (e.g. because of *keepDistribution*)
   then a comment doesn't count as "have seen that line".

This means that if we get to the end of the file, with only commented-
out GRUB_DISTRIBUTION lines, and *keepDistribution* is set, then we'll
still write a distribution line -- because otherwise it's not set at all.

src/modules/grubcfg/main.py

@@ -180,7 +180,11 @@ def modify_grub_default(partitions, root_mount_point, distributor):
                   or lines[i].startswith("GRUB_DISTRIBUTOR")):
                 if libcalamares.job.configuration.get("keepDistributor", false):
                     lines[i] = distributor_line
-                have_distributor_line = True
+                    have_distributor_line = True
+                else:
+                    # We're not updating because of *keepDistributor*, but if
+                    # this was a comment line, then it's still not been set.
+                    have_distributor_line = not lines[i].startsdwith("#")
     else:
         lines = []