t1006: extract --batch-command inputs to variables

t1006: extract --batch-command inputs to variables

A future commit will want to ensure that various `--batch`-related
options produce the same output whether their input is newline
terminated, or NUL terminated (and a to-be-implemented `-z` option
exists).

To prepare for this, extract the given input(s) into separate variables
to that their LF characters can easily be converted into NUL bytes when
testing the new `-z` mode.

This is consistent with other tests in t1006 (which these days is no
longer a shining example of our CodingGuidelines).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>