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

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);