}
 
 static int show_patch_diff(struct combine_diff_path *elem, int num_parent,
-                          int dense, const char *header)
+                          int dense, const char *header,
+                          struct diff_options *opt)
 {
        unsigned long size, cnt, lno;
        char *result, *cp, *ep;
        char ourtmp_buf[TMPPATHLEN];
        char *ourtmp = ourtmp_buf;
        int working_tree_file = !memcmp(elem->sha1, null_sha1, 20);
+       int abbrev = opt->full_index ? 40 : DEFAULT_ABBREV;
 
        /* Read the result of merge first */
        if (!working_tree_file) {
                printf("index ");
                for (i = 0; i < num_parent; i++) {
                        abb = find_unique_abbrev(elem->parent[i].sha1,
-                                                DEFAULT_ABBREV);
+                                                abbrev);
                        printf("%s%s", i ? "," : "", abb);
                }
-               abb = find_unique_abbrev(elem->sha1, DEFAULT_ABBREV);
+               abb = find_unique_abbrev(elem->sha1, abbrev);
                printf("..%s\n", abb);
 
                if (mode_differs) {
 
        default:
        case DIFF_FORMAT_PATCH:
-               return show_patch_diff(p, num_parent, dense, header);
+               return show_patch_diff(p, num_parent, dense, header, opt);
        }
 }