t/README: add a note about don't saving stderr of compound commands

Explain in 't/README' why it is a bad idea to redirect and verify the
stderr of compound commands, in the hope that future contributions
will follow this advice and the test suite will keep working with '-x'
tracing and /bin/sh.

While at it, since we can now run the test suite with '-x' without
needing a Bash version supporting BASH_XTRACEFD, remove the now
outdated caution note about non-Bash shells from the description of
the '-x' option.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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