branch,checkout: fix --track usage strings

branch,checkout: fix --track usage strings

As Ævar pointed out in [1], the use of PARSE_OPT_LITERAL_ARGHELP with a
list of allowed parameters is not recommended. Both git-branch and
git-checkout were changed in d311566 (branch: add flags and config to
inherit tracking, 2021-12-20) to use this discouraged combination for
their --track flags.

Fix this by removing PARSE_OPT_LITERAL_ARGHELP, and changing the arghelp
to simply be "mode". Users may discover allowed values in the manual
pages.

[1]: https://lore.kernel.org/git/220111.86a6g3yqf9.gmgdl@evledraar.gmail.com/

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>