Separate GIT_TRACE description into what it prints and how to configure
where trace output is printed to. Change other GIT_TRACE_* descriptions to
refer to GIT_TRACE.
Add descriptions for GIT_TRACE_SETUP and GIT_TRACE_SHALLOW.
Signed-off-by: Karsten Blees <blees@dcon.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>