t6300-for-each-ref: fix "more than one quoting style" tests
'git for-each-ref' should error out when invoked with more than one
quoting style options. The tests checking this have two issues:
- They run 'git for-each-ref' upstream of a pipe, hiding its exit
code, thus don't actually checking that 'git for-each-ref' exits
with error code.
- They check the error message in a rather roundabout way.
Ensure that 'git for-each-ref' exits with an error code using the
'test_must_fail' helper function, and check its error message by
grepping its saved standard error.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>