t: prefer "git config --file" to GIT_CONFIG with test_must_fail

This lets us get rid of an extra "env" invocation in the
middle, and is slightly more readable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1300-repo-config.sh

@@ -961,15 +961,15 @@ test_expect_success SYMLINKS 'symlinked configuration' '
 '
 
 test_expect_success 'nonexistent configuration' '
-	test_must_fail env GIT_CONFIG=doesnotexist git config --list &&
-	test_must_fail env GIT_CONFIG=doesnotexist git config test.xyzzy
+	test_must_fail git config --file=doesnotexist --list &&
+	test_must_fail git config --file=doesnotexist test.xyzzy
 '
 
 test_expect_success SYMLINKS 'symlink to nonexistent configuration' '
 	ln -s doesnotexist linktonada &&
 	ln -s linktonada linktolinktonada &&
-	test_must_fail env GIT_CONFIG=linktonada git config --list &&
-	test_must_fail env GIT_CONFIG=linktolinktonada git config --list
+	test_must_fail git config --file=linktonada --list &&
+	test_must_fail git config --file=linktolinktonada --list
 '
 
 test_expect_success 'check split_cmdline return' "