test-lib: silence "-x" cleanup under bash

test-lib: silence "-x" cleanup under bash

When the test suite's "-x" option is used with bash, we end
up seeing cleanup cruft in the output:

  $ bash t0001-init.sh -x
  [...]
  ++ diff -u expected actual
  + test_eval_ret_=0
  + want_trace
  + test t = t
  + test t = t
  + set +x
  ok 42 - re-init from a linked worktree

This ranges from mildly annoying (for a successful test) to
downright confusing (when we say "last command exited with
error", but it's really 5 commands back).

We normally are able to suppress this cleanup. As the
in-code comment explains, we can't convince the shell not to
print it, but we can redirect its stderr elsewhere.

But since d88785e424 (test-lib: set BASH_XTRACEFD
automatically, 2016-05-11), that doesn't hold for bash. It
sends the "set -x" output directly to descriptor 4, not to
stderr.

We can fix this by also redirecting descriptor 4, and
paying close attention to which commands redirected and
which are not (see the updated comment).

Two alternatives I considered and rejected:

  - unsetting and setting BASH_XTRACEFD; doing so closes the
    descriptor, which we must avoid

  - we could keep everything in a single block as before,
    redirect 4>/dev/null there, but retain 5>&4 as a copy.
    And then selectively restore 4>&5 for commands which
    should be allowed to trace. This would work, but the
    descriptor swapping seems unnecessarily confusing.

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