t40xx: don't use iconv(1) without ICONV prereq

t40xx: don't use iconv(1) without ICONV prereq

We've got a couple of tests related to diffs in t40xx that use the
iconv(1) executable to convert the encoding of a commit message. All of
these tests are prepared to handle a missing ICONV prereq, in which case
they will simply use UTF-8 encoding.

But even if the ICONV prerequisite has failed we try to use the iconv(1)
executable, even though it's not safe to assume that the executable
exists in that case. And besides that, it's also unnecessary to use
iconv(1) in the first place, as we would only use it to convert from
UTF-8 to UTF-8, which should be equivalent to a no-op.

Fix the issue and skip the call to iconv(1) in case the prerequisite is
not set. This makes tests work on systems that don't have iconv at all.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>