t5552: suppress upload-pack trace output

t5552: suppress upload-pack trace output

The t5552 test script uses GIT_TRACE_PACKET to monitor what
git-fetch sends and receives. However, because we're
accessing a local repository, the child upload-pack also
sends trace output to the same file.

On Linux, this works out OK. We open the trace file with
O_APPEND, so all writes are atomically positioned at the end
of the file. No data can be overwritten or omitted. And
since we prepare our small writes in a strbuf and write them
with a single write(), we should see each line as an atomic
unit. The order of lines between the two processes is
undefined, but the test script greps only for "fetch>" or
"fetch<" lines. So under Linux, the test results are
deterministic.

The test fails intermittently on Windows, however,
reportedly even overwriting bits of the output file (i.e.,
O_APPEND does not seem to give us an atomic position+write).

Since the test only cares about the trace output from fetch,
we can just disable the output from upload-pack. That
doesn't solve the greater question of O_APPEND/trace issues
under Windows, but it easily fixes the flakiness from this
test.

Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>