MINOR: trace: don't call strlen() on the thread-id numeric encoding

In __trace(), we're making an integer for the thread id but this one
is passed through strlen() in the call to ist() because it's not a
constant. We do know that it's exactly 3 chars long so we can manage
this using ist2() and pass it the length instead in order to reduce
the number of calls to strlen().

Also let's note that the thread number will no longer be numeric for
thread numbers above 100.

src/trace.c

@@ -285,7 +285,7 @@ void __trace(enum trace_level level, uint64_t mask, struct trace_source *src,
 	tnum[1] = '0' + tid % 10;
 	tnum[2] = '|';
 	tnum[3] = 0;
-	line[words++] = ist(tnum);
+	line[words++] = ist2(tnum, 3);
 	line[words++] = src->name;
 	line[words++] = ist("|");
 	line[words++] = ist2("012345" + level, 1); // "0" to "5"