Trace into open fd and refactor tracing code.

Now if GIT_TRACE is set to an integer value greater than 1
and lower than 10, we interpret this as an open fd value
and we trace into it. Note that this behavior is not
compatible with the previous one.

We also trace whole messages using one write(2) call to
make sure messages from processes do net get mixed up in
the middle.

It's now possible to run the tests like this:

	GIT_TRACE=9 make test 9>/var/tmp/trace.log

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>