t3700-add: fix dependence on stdout and stderr buffering

t3700-add: fix dependence on stdout and stderr buffering

One test case checked the stdout and stderr of 'git add' by constructing a
single 'expect' file that contained both streams. But when the command
runs, the order of stdout and stderr output is unpredictable because it
depends on how the streams are buffered. At least on Windows, the buffering
is different from what the test case expected. Hence, check the two output
texts separately.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Acked-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>