builtin/config: check for writeability after source is set up

The `check_write()` function verifies that we do not try to write to a
config source that cannot be written to, like for example stdin. But
while the new subcommands do call this function, they do so before
calling `handle_config_location()`. Consequently, we only end up
checking the default config location for writeability, not the location
that was actually specified by the caller of git-config(1).

Fix this by calling `check_write()` after `handle_config_location()`. We
will further clarify the relationship between those two functions in a
subsequent commit where we remove the global state that both implicitly
rely on.

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

t/t1300-config.sh

@@ -2835,6 +2835,12 @@ test_expect_success 'specifying multiple modes causes failure' '
 	test_cmp expect err
 '
 
+test_expect_success 'writing to stdin is rejected' '
+	echo "fatal: writing to stdin is not supported" >expect &&
+	test_must_fail git config ${mode_set} --file - foo.bar baz 2>err &&
+	test_cmp expect err
+'
+
 done
 
 test_done