t: redirect stderr GIT_TRACE to descriptor 4

If you run a test script like:

  GIT_TRACE=1 ./t0061-run-command.sh

you may get test failures, because some tests capture and
check the stderr output from git commands (and with
GIT_TRACE set to 1, the trace output will be included
there).

When we see GIT_TRACE set like this, we print a warning to
the user. However, we can do even better than that by just
pointing it to descriptor 4, which all tests leave connected
to the test script's stderr. That's likely what the user
intended (and any scripts that do want to see GIT_TRACE
output will set GIT_TRACE themselves).

Not only does this avoid false negatives in the tests, but
it means the user will actually see trace output for git
calls that redirect their stderr (whereas before, it was
sometimes confusingly buried in a file).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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