t5503: GIT_DEBUG_SEND_PACK is not supported on MinGW

t5503: GIT_DEBUG_SEND_PACK is not supported on MinGW

The test opens fd 3 and instructs git-upload-pack (via GIT_DEBUG_SEND_PACK)
to log information to that channel.

The way in which new processes are spawned by git on MinGW does not inherit
all file descriptors to the child processes, but only 0, 1, and 2.
The tests in t5503 require that file descriptor 3 is inherited from
git-fetch to git-upload-pack.

A complete implementation is non-trivial and not warranted just to satisfy
this test.  Note that the incompleteness applies only to the executables
that use compat/mingw.c; bash and perl (the other important executables
used by git) are complete, of course.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>