[grubcfg] Guard against stupid configurations

- Scenario: *keepDistribution* is true, and the existing file contains
   a GRUB_DISTRIBUTION line **followed** by a commented-out GRUB_DISTRIBUTION
   line.
 - In that case, the commented-out line would change the flag back to
   False, and we'd end up writing a second GRUB_DISTRIBUTION line at the end.

Prevent that: the flag can only go to "True" and then stays there.

Editorial: If your grub configuration would have tripped this up, then
you're doing something wrong. Clean up the configuration file first.

src/modules/grubcfg/main.py

@@ -184,7 +184,7 @@ def modify_grub_default(partitions, root_mount_point, distributor):
                 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("#")
+                    have_distributor_line = have_distributor_line or not lines[i].startsdwith("#")
     else:
         lines = []