sha1_name: check @{-N} errors sooner

It's trivial to check for them in the @{N} parsing loop.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -448,7 +448,10 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	if (len && str[len-1] == '}') {
 		for (at = len-4; at >= 0; at--) {
 			if (str[at] == '@' && str[at+1] == '{') {
-				if (at == 0 && str[2] == '-') {
+				if (str[at+2] == '-') {
+					if (at != 0)
+						/* @{-N} not at start */
+						return -1;
 					nth_prior = 1;
 					continue;
 				}
@@ -497,10 +500,6 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 		unsigned long co_time;
 		int co_tz, co_cnt;
 
-		/* a @{-N} placed anywhere except the start is an error */
-		if (str[at+2] == '-')
-			return -1;
-
 		/* Is it asking for N-th entry, or approxidate? */
 		for (i = nth = 0; 0 <= nth && i < reflog_len; i++) {
 			char ch = str[at+2+i];