t/check-non-portable-shell: make error messages more compact

t/check-non-portable-shell: make error messages more compact

Error messages emitted by this linting script are long and noisy,
consisting of several sections:

    <test-script>:<line#>: error: <explanation>: <failed-shell-text>

The line of failed shell text, usually coming from within a test body,
is often indented by one or two TABs, with the result that the actual
(important) text is separated from <explanation> by a good deal of empty
space. This can make for a difficult read, especially on typical
80-column terminals.

Make the messages more compact and perhaps a bit easier to digest by
folding out the leading whitespace from <failed-shell-text>.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>