t4205: improve handling of ICONV prerequisite

t4205: improve handling of ICONV prerequisite

In t4205 we have a bunch of tests that depend on the iconv prereq. This
is for most of the part because we format commit messages that have been
encoded in an encoding different than UTF-8.

Those tests fall into two classes though:

  - One class of tests outputs the data as-is without reencoding.

  - One class of tests outputs the data with "i18n.logOutputEncoding" to
    reencode it.

Curiously enough, both of these classes are marked with the ICONV
prereq, even though one might expect that the first class wouldn't need
the prereq. This is because we unconditionally use ISO-8859-1 encoding
for the initial commit message, and thus we depend on converting to
UTF-8 indeed.

This creates another problem though: when the iconv(1) executable does
not exist the test setup fails, even in the case where the ICONV prereq
has not been set.

Fix these issues by making the test encoding conditional on ICONV: if
it's available we use ISO-8859-1, otherwise we use UTF-8. This fixes the
test setup on platforms without iconv(1), and it allows us to drop the
ICONV prereq from a bunch of tests.

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