tag: use a "goto cleanup" pattern, leak less memory

tag: use a "goto cleanup" pattern, leak less memory

Change cmd_tag() to free its "struct strbuf"'s instead of using an
UNLEAK() pattern. This changes code added in 886e1084d78 (builtin/:
add UNLEAKs, 2017-10-01).

As shown in the context of the declaration of the "struct
msg_arg" (which I'm changing to use a designated initializer while at
it, and to show the context in this change), that struct is just a
thin wrapper around an int and "struct strbuf".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>