[PATCH 2/2] tests: fix non-portable iconv invocation

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

The iconv that comes with a FreeBSD 11.2-RELEASE-p2 box I have access
to doesn't support the SHIFT-JIS encoding. Guard a test added in
e92d62253 ("convert: add round trip check based on
'core.checkRoundtripEncoding'", 2018-04-15) first released with Git
v2.18.0 with a prerequisite that checks for its availability.

The iconv command is in POSIX, and we have numerous tests
unconditionally relying on its ability to convert ASCII, UTF-8 and
UTF-16, but unconditionally relying on the presence of more obscure
encodings isn't portable.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 t/t0028-working-tree-encoding.sh | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
index 12b8eb963a..b0f4490e8e 100755
--- a/t/t0028-working-tree-encoding.sh
+++ b/t/t0028-working-tree-encoding.sh
@@ -203,7 +203,11 @@ test_expect_success 'error if encoding garbage is already in Git' '
 	test_i18ngrep "error: BOM is required" err.out
 '
 
-test_expect_success 'check roundtrip encoding' '
+test_lazy_prereq ICONV_SHIFT_JIS '
+	iconv -f UTF-8 -t SHIFT-JIS </dev/null 2>/dev/null
+'
+
+test_expect_success ICONV_SHIFT_JIS 'check roundtrip encoding' '
 	test_when_finished "rm -f roundtrip.shift roundtrip.utf16" &&
 	test_when_finished "git reset --hard HEAD" &&
 
-- 
2.19.0.rc0.228.g281dcd1b4d0