grep -An -Bm: fix invocation of external grep command

When building command line to invoke external grep, the
arguments to -A/-B/-C options were placd in randarg[] buffer,
but the code forgot that snprintf() does not count terminating
NUL in its return value.  This caused "git grep -A1 -B2" to
invoke external grep with "-B21 -A1".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -294,7 +294,7 @@ static int external_grep(struct grep_opt *opt, const char **paths, int cached)
 			if (opt->pre_context) {
 				push_arg("-B");
 				len += snprintf(argptr, sizeof(randarg)-len,
-						"%u", opt->pre_context);
+						"%u", opt->pre_context) + 1;
 				if (sizeof(randarg) <= len)
 					die("maximum length of args exceeded");
 				push_arg(argptr);
@@ -303,7 +303,7 @@ static int external_grep(struct grep_opt *opt, const char **paths, int cached)
 			if (opt->post_context) {
 				push_arg("-A");
 				len += snprintf(argptr, sizeof(randarg)-len,
-						"%u", opt->post_context);
+						"%u", opt->post_context) + 1;
 				if (sizeof(randarg) <= len)
 					die("maximum length of args exceeded");
 				push_arg(argptr);
@@ -313,7 +313,7 @@ static int external_grep(struct grep_opt *opt, const char **paths, int cached)
 		else {
 			push_arg("-C");
 			len += snprintf(argptr, sizeof(randarg)-len,
-					"%u", opt->post_context);
+					"%u", opt->post_context) + 1;
 			if (sizeof(randarg) <= len)
 				die("maximum length of args exceeded");
 			push_arg(argptr);