Do not misidentify "git merge foo HEAD" as an old-style invocation

This was misinterpreted as an ancient style "git merge <message> HEAD
<commit> <commit>..." that merges one (or more) <commit> into the current
branch and record the resulting commit with the given message.  Then a
later sanity check found that there is no <commit> specified and gave
a usage message.

Tested-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge.c

@@ -792,7 +792,7 @@ static int suggest_conflicts(void)
 static struct commit *is_old_style_invocation(int argc, const char **argv)
 {
 	struct commit *second_token = NULL;
-	if (argc > 1) {
+	if (argc > 2) {
 		unsigned char second_sha1[20];
 
 		if (get_sha1(argv[1], second_sha1))