t9500: use test_config to set/unset git config variables

Instead of using construct such as:
    test_when_finished "git config --unset <key>"
    git config <key> <value>
uses
    test_config <key> <value>
The latter takes care of removing <key> at the end of the test.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9500-gitweb-standalone-no-errors.sh

@@ -539,8 +539,7 @@ test_expect_success \
 	 test_when_finished "GIT_COMMITTER_NAME=\"C O Mitter\"" &&
 	 echo "ISO-8859-1" >> file &&
 	 git add file &&
-	 git config i18n.commitencoding ISO-8859-1 &&
-	 test_when_finished "git config --unset i18n.commitencoding" &&
+	 test_config i18n.commitencoding ISO-8859-1 &&
 	 git commit -F "$TEST_DIRECTORY"/t3900/ISO8859-1.txt &&
 	 gitweb_run "p=.git;a=commit"'