t5615: avoid re-using descriptor 4

t5615: avoid re-using descriptor 4

File descriptors 3 and 4 are special in our test suite, as
they link back to the test script's original stdout and
stderr. Normally this isn't something tests need to worry
about: they are free to clobber these descriptors for
sub-commands without affecting the overall script.

But there's one very special thing about descriptor 4: since
d88785e424 (test-lib: set BASH_XTRACEFD automatically,
2016-05-11), we ask bash to output "set -x" output to it by
number. This goes to _any_ descriptor 4, even if it no
longer points to the place it did when we set BASH_XTRACEFD.

But in t5615, we run a shell loop with descriptor 4
redirected.  As a result, t5615 works with non-bash shells
even with "-x". And it works with bash without "-x". But the
combination of "bash t5615-alternate-env.sh -x" gets a test
failure (because our "set -x" output pollutes one of the
files).

We can fix this by using any descriptor _except_ the magical
4. So let's switch arbitrarily to using 5/6 in this loop,
not 3/4.

Another alternative is to use a different descriptor for
BASH_XTRACEFD. But picking an unused one turns out to be
hard. Most shells limit us to 9 numbered descriptors. Bash
can handle more, but:

  - while the BASH_XTRACEFD is specific to bash, GIT_TRACE=4
    has a similar problem, and would affect all shells

  - constructs like "999>/dev/null" are synticatically
    invalid to non-bash shells. So we have to actually bury
    it inside an eval, which creates more complications.

Of the numbers 1-9, you might think that "9" would be less
used than "4". But it's not; many of our scripts use
descriptors 8 and 9 (probably under the assumption that they
are high and therefore unused). The least-used descriptor is
currently "7". We could switch to that, but we're just
trading one magic number for another.

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