Trace into a file or an open fd and refactor tracing code.

Trace into a file or an open fd and refactor tracing code.

If GIT_TRACE is set to an absolute path (starting with a
'/' character), we interpret this as a file path and we
trace into it.

Also 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.

This patch makes it possible to get trace information when
running "make test".

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