t1901: adjust nul format output instead of expected value

t1901: adjust nul format output instead of expected value

The test 'keyvalue and nul format', as it description says, test both
`keyvalue` and `nul` format. These formats are similar, differing only in
their field separator (= in the former, LF in the latter) and their
record separator (LF in the former, NUL in the latter). This way, both
formats can be tested using the same expected output and only replacing
the separators in one of the output formats.

However, it is not desirable to have a NUL character in the files
compared by test_cmp because, if that assetion fails, diff will consider
them binary files and won't display the differences properly.

Adjust the output of `git repo structure --format=nul` in t1901, matching the
--format=keyvalue ones. Compare this output against the same value expected
from --format=keyvalue, without using files with NUL characters in
test_cmp.

Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>