}
        }
 
-       if (have_option_m) {
+       if (have_option_m && !fixup_message) {
                strbuf_addbuf(&sb, &message);
                hook_arg1 = "message";
        } else if (logfile && !strcmp(logfile, "-")) {
                ctx.output_encoding = get_commit_output_encoding();
                format_commit_message(commit, "fixup! %s\n\n",
                                      &sb, &ctx);
+               if (have_option_m)
+                       strbuf_addbuf(&sb, &message);
                hook_arg1 = "message";
        } else if (!stat(git_path_merge_msg(), &statbuf)) {
                /*
                f++;
        if (f > 1)
                die(_("Only one of -c/-C/-F/--fixup can be used."));
-       if (have_option_m && f > 0)
-               die((_("Option -m cannot be combined with -c/-C/-F/--fixup.")));
+       if (have_option_m && (edit_message || use_message || logfile))
+               die((_("Option -m cannot be combined with -c/-C/-F.")));
        if (f || have_option_m)
                template_file = NULL;
        if (edit_message)
        rev.show_root_diff = 1;
        get_commit_format(format.buf, &rev);
        rev.always_show_header = 0;
-       rev.diffopt.detect_rename = 1;
+       rev.diffopt.detect_rename = DIFF_DETECT_RENAME;
        rev.diffopt.break_opt = 0;
        diff_setup_done(&rev.diffopt);
 
                                allow_fast_forward = 0;
                }
                if (allow_fast_forward)
-                       parents = reduce_heads(parents);
+                       reduce_heads_replace(&parents);
        } else {
                if (!reflog_msg)
                        reflog_msg = (whence == FROM_CHERRY_PICK)