shredder: only hash single-inode groups that are actually hardlinked

`group->n_inodes == 1` is also true for groups that simply consist of a
single file. This condition will cause all single-file groups to be
hashed if `--merge-directories` is also set.

Additionally, the whole `group->n_inodes == 1` condition is redundant
because not following on the branch means that `group->n_clusters == 1`
and therefore `group->n_inodes == 1`.

Thus, check that we are actually dealing with a group of hardlinks (and
not just a single-file group that did not cause an early return because
we are also doing `--hash-unmatched`).

Fixes #614.

lib/shredder.c

@@ -855,7 +855,7 @@ static void rm_shred_group_update_status(RmShredGroup *group) {
     } else if(group->n_clusters > 1) {
         // we have potential match candidates; start hashing
         group->status = RM_SHRED_GROUP_START_HASHING;
-    } else if(group->n_inodes == 1 && group->n_unhashed_clusters > 0 &&
+    } else if(group->num_files > 1 && group->n_unhashed_clusters > 0 &&
               group->session->cfg->merge_directories) {
         /* special case of hardlinked files that still need hashing to help identify
          * matching directories */