MINOR: trace: extend the source location to 13 chars

With 4-digit line numbers, this allows to emit up to 6 chars of file
name before extension, instead of 3 previously.
diff --git a/src/trace.c b/src/trace.c
index e8de89b..8d68b66 100644
--- a/src/trace.c
+++ b/src/trace.c
@@ -178,9 +178,9 @@
 	 */
 	line[0] = ist("[");
 	line[1] = where;
-	if (line[1].len > 10) {
-		line[1].ptr += (line[1].len - 10);
-		line[1].len = 10;
+	if (line[1].len > 13) {
+		line[1].ptr += (line[1].len - 13);
+		line[1].len = 13;
 	}
 	line[2] = ist("] ");