merge: --log appends shortlog to message if specified

When the user specifies a message, use fmt_merge_msg_shortlog() to
append the shortlog.

Previously, when a message was specified, we ignored the merge title
("Merge <foo> into <bar>") and shortlog from fmt_merge_msg().

Update the documentation for -m to reflect this too.
Signed-off-by: NTay Ray Chuan <rctay89@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -58,7 +58,12 @@ include::merge-options.txt[]
 
 -m <msg>::
 	Set the commit message to be used for the merge commit (in
-	case one is created). The 'git fmt-merge-msg' command can be
+	case one is created).
+
+	If `--log` is specified, a shortlog of the commits being merged
+	will be appended to the specified message.
+
+	The 'git fmt-merge-msg' command can be
 	used to give a good default for automated 'git merge'
 	invocations.
 

builtin/fmt-merge-msg.c

@@ -298,6 +298,9 @@ static int do_fmt_merge_msg(int merge_title, int merge_summary,
 		rev.ignore_merges = 1;
 		rev.limited = 1;
 
+		if (suffixcmp(out->buf, "\n"))
+			strbuf_addch(out, '\n');
+
 		for (i = 0; i < origins.nr; i++)
 			shortlog(origins.items[i].string, origins.items[i].util,
 					head, &rev, limit, out);

builtin/merge.c

@@ -981,18 +981,22 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		/*
 		 * All the rest are the commits being merged;
 		 * prepare the standard merge summary message to
-		 * used as the merge message.  If remote
+		 * be appended to the given message.  If remote
 		 * is invalid we will die later in the common
 		 * codepath so we discard the error in this
 		 * loop.
 		 */
-		if (!have_message) {
-			for (i = 0; i < argc; i++)
-				merge_name(argv[i], &merge_names);
+		for (i = 0; i < argc; i++)
+			merge_name(argv[i], &merge_names);
+
+		if (have_message && option_log)
+			fmt_merge_msg_shortlog(&merge_names, &merge_msg);
+		else if (!have_message)
 			fmt_merge_msg(option_log, &merge_names, &merge_msg);
-			if (merge_msg.len)
-				strbuf_setlen(&merge_msg, merge_msg.len-1);
-		}
+
+
+		if (!(have_message && !option_log) && merge_msg.len)
+			strbuf_setlen(&merge_msg, merge_msg.len-1);
 	}
 
 	if (head_invalid || !argc)

t/t7604-merge-custom-message.sh

@@ -40,7 +40,7 @@ test_expect_success 'merge c2 with a custom message' '
 	test_cmp exp.subject actual
 '
 
-test_expect_failure 'merge --log appends to custom message' '
+test_expect_success 'merge --log appends to custom message' '
 	git reset --hard c1 &&
 	git merge --log -m "$(cat exp.subject)" c2 &&
 	git cat-file commit HEAD | sed -e "1,/^$/d" >actual &&