repart: keep foreign partitions' UUID and label across partition table reloads

With --size=auto, repart unloads and reloads the partition table after
context_acquire_partition_uuids_and_labels() ran. The reload recreates the
foreign partitions (those without a definition) with a null new UUID and
no new label, so context_mangle_partitions() sees them as changed and hits
assert(!PARTITION_IS_FOREIGN(p)) when adding a partition to an existing
image:

  Initializing UUID of existing partition 0.
  Setting partition label of existing partition 0.
  Assertion '!PARTITION_IS_FOREIGN(p)' failed.

Set the new UUID and label of a foreign partition when it is loaded, so a
reload keeps them.

(cherry picked from commit 4fc4f6bac772ff3749ba7e0bbcd607175694420c)

src/repart/repart.c

@@ -4130,6 +4130,14 @@ static int context_load_partition_table(Context *context) {
                         np->partno = partno;
                         np->current_label = TAKE_PTR(label_copy);
 
+                        /* Never touch foreign partitions: keep their UUID and label. Set here rather than in
+                         * context_acquire_partition_uuids_and_labels(), as --size=auto reloads the partition
+                         * table after that ran, which recreates the foreign partitions. */
+                        np->new_uuid = id;
+                        r = strdup_to(&np->new_label, np->current_label);
+                        if (r < 0)
+                                return log_oom();
+
                         np->current_partition = p;
                         sym_fdisk_ref_partition(p);
 
@@ -7863,18 +7871,9 @@ static int context_acquire_partition_uuids_and_labels(Context *context) {
         LIST_FOREACH(partitions, p, context->partitions) {
                 sd_id128_t uuid;
 
-                /* Never touch foreign partitions */
-                if (PARTITION_IS_FOREIGN(p)) {
-                        p->new_uuid = p->current_uuid;
-
-                        if (p->current_label) {
-                                r = free_and_strdup_warn(&p->new_label, strempty(p->current_label));
-                                if (r < 0)
-                                        return r;
-                        }
-
+                /* Never touch foreign partitions, their UUID and label are set when loading them. */
+                if (PARTITION_IS_FOREIGN(p))
                         continue;
-                }
 
                 (void) context_notify(context, PROGRESS_ACQUIRING_PARTITION_LABELS, p->definition_path, UINT_MAX);