show-branch: use a strbuf for reflog descriptions

When we show "branch@{0}", we format into a fixed-size
buffer using sprintf. This can overflow if you have long
branch names. We can fix it by using a temporary strbuf.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/show-branch.c

diff --git a/builtin/show-branch.c b/builtin/show-branch.c
index 270e39c..9e60b12 100644
--- a/builtin/show-branch.c
+++ b/builtin/show-branch.c
@@ -720,7 +720,6 @@
 
 	if (reflog) {
 		unsigned char sha1[20];
-		char nth_desc[256];
 		char *ref;
 		int base = 0;
 		unsigned int flags = 0;
@@ -759,6 +758,7 @@
 
 		for (i = 0; i < reflog; i++) {
 			char *logmsg;
+			char *nth_desc;
 			const char *msg;
 			unsigned long timestamp;
 			int tz;
@@ -777,8 +777,10 @@
 						show_date(timestamp, tz, 1),
 						msg);
 			free(logmsg);
-			sprintf(nth_desc, "%s@{%d}", *av, base+i);
+
+			nth_desc = xstrfmt("%s@{%d}", *av, base+i);
 			append_ref(nth_desc, sha1, 1);
+			free(nth_desc);
 		}
 		free(ref);
 	}