t0028: extract encoding helpers to lib-encoding.sh

The following patch will add tests outside t0028 which will also need to
re-encode some strings. Extract the auxiliary encoding functions from
t0028 to a common lib file so that they can be reused.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-encoding.sh

@@ -0,0 +1,25 @@
+# Encoding helpers
+
+test_lazy_prereq NO_UTF16_BOM '
+	test $(printf abc | iconv -f UTF-8 -t UTF-16 | wc -c) = 6
+'
+
+test_lazy_prereq NO_UTF32_BOM '
+	test $(printf abc | iconv -f UTF-8 -t UTF-32 | wc -c) = 12
+'
+
+write_utf16 () {
+	if test_have_prereq NO_UTF16_BOM
+	then
+		printf '\376\377'
+	fi &&
+	iconv -f UTF-8 -t UTF-16
+}
+
+write_utf32 () {
+	if test_have_prereq NO_UTF32_BOM
+	then
+		printf '\0\0\376\377'
+	fi &&
+	iconv -f UTF-8 -t UTF-32
+}

t/t0028-working-tree-encoding.sh

@@ -6,33 +6,10 @@ GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY/lib-encoding.sh"
 
 GIT_TRACE_WORKING_TREE_ENCODING=1 && export GIT_TRACE_WORKING_TREE_ENCODING
 
-test_lazy_prereq NO_UTF16_BOM '
-	test $(printf abc | iconv -f UTF-8 -t UTF-16 | wc -c) = 6
-'
-
-test_lazy_prereq NO_UTF32_BOM '
-	test $(printf abc | iconv -f UTF-8 -t UTF-32 | wc -c) = 12
-'
-
-write_utf16 () {
-	if test_have_prereq NO_UTF16_BOM
-	then
-		printf '\376\377'
-	fi &&
-	iconv -f UTF-8 -t UTF-16
-}
-
-write_utf32 () {
-	if test_have_prereq NO_UTF32_BOM
-	then
-		printf '\0\0\376\377'
-	fi &&
-	iconv -f UTF-8 -t UTF-32
-}
-
 test_expect_success 'setup test files' '
 	git config core.eol lf &&