merge script: handle -m --log correctly

Based on v1.7.1.1~23^2 (merge: --log appends shortlog to message if
specified, 2010-05-11).  Without this change, the scripted
(non-builtin) merge does not pass t7604.

Cc: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-merge.sh

@@ -294,12 +294,18 @@ else
 	# the given message.  If remote is invalid we will die
 	# later in the common codepath so we discard the error
 	# in this loop.
-	merge_name=$(for remote
+	merge_msg="$(
+		for remote
 		do
 			merge_name "$remote"
-		done | git fmt-merge-msg $log_arg
-	)
-	merge_msg="${merge_msg:+$merge_msg$LF$LF}$merge_name"
+		done |
+		if test "$have_message" = t
+		then
+			git fmt-merge-msg -m "$merge_msg" $log_arg
+		else
+			git fmt-merge-msg $log_arg
+		fi
+	)"
 fi
 head=$(git rev-parse --verify "$head_arg"^0) || usage