clone: make it possible to specify --tags

clone: make it possible to specify --tags

Option --no-tags was added in 0dab2468ee (clone: add a --no-tags option
to clone without tags, 2017-04-26). At the time there was no need to
support --tags as well, although there was some conversation about
it[1].

To simplify the code and to prepare for future commits, invert the flag
internally. Functionally there is no change, because the flag is
default-enabled passing `--tags` has no effect, so there's no need to
add tests for this.

[1]: https://lore.kernel.org/git/CAGZ79kbHuMpiavJ90kQLEL_AR0BEyArcZoEWAjPPhOFacN16YQ@mail.gmail.com/

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>