trace: add pid to each output line

When there are many git commands running, it can be
confusing which process is producing which message. And if
one is trying to correlate a specific invocation with a
trace-file created using "%p", it's rather impossible.

Let's print the pid alongside each line. This does extend
the already-wide trace lines. We could make it optional, but
it's probably not worth the complexity of extra knobs.
Looking at trace output is already an exceptional thing to
be doing, and verbose output is not going to bother anyone.

Signed-off-by: Jeff King <peff@peff.net>

Author: peff

trace.c

@@ -129,8 +129,9 @@ static int prepare_trace_line(const char *file, int line,
 	localtime_r(&secs, &tm);
 	strbuf_addf(buf, "%02d:%02d:%02d.%06ld %s:%d", tm.tm_hour, tm.tm_min,
 		    tm.tm_sec, (long) tv.tv_usec, file, line);
-	/* align trace output (column 40 catches most files names in git) */
-	while (buf->len < 40)
+	strbuf_addf(buf, "[pid=%lu] ", (unsigned long)getpid());
+	/* align trace output (column 50 catches most files names in git) */
+	while (buf->len < 50)
 		strbuf_addch(buf, ' ');
 
 	return 1;