trace: disable key after write error

trace: disable key after write error

If we get a write error writing to a trace descriptor, the
error isn't likely to go away if we keep writing. Instead,
you'll just get the same error over and over. E.g., try:

  GIT_TRACE_PACKET=42 git ls-remote >/dev/null

You don't really need to see:

  warning: unable to write trace for GIT_TRACE_PACKET: Bad file descriptor

hundreds of times. We could fallback to tracing to stderr,
as we do in the error code-path for open(), but there's not
much point. If the user fed us a bogus descriptor, they're
probably better off fixing their invocation. And if they
didn't, and we saw a transient error (e.g., ENOSPC writing
to a file), it probably doesn't help anybody to have half of
the trace in a file, and half on stderr.

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