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: NKarsten Blees <blees@dcon.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>