Documentation/git.txt: improve documentation of 'GIT_TRACE*' variables
Documentation/git.txt: improve documentation of 'GIT_TRACE*' variables

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 <955b63282c975ba379c4995cac1c2e1e7dfebe62@dcon.de>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git