parseopt: handle malformed --expire arguments more nicely

A few commands that parse --expire=<time> command line option behave
sillily when given nonsense input.  For example

    $ git prune --no-expire
    Segmentation falut
    $ git prune --expire=npw; echo $?
    129

Both come from parse_opt_expiry_date_cb().

The former is because the function is not prepared to see arg==NULL
(for "--no-expire", it is a norm; "--expire" at the end of the
command line could be made to pass NULL, if it is told that the
argument is optional, but we don't so we do not have to worry about
that case).

The latter is because it does not check the value returned from the
underlying parse_expiry_date().

This seems to be a recent regression introduced while we attempted
to avoid spewing the entire usage message when given a correct
option but with an invalid value at 3bb0923f ("parse-options: do not
show usage upon invalid option value", 2018-03-22).  Before that, we
didn't fail silently but showed a full usage help (which arguably is
not all that better).

Also catch this error early when "git gc --prune=<expiration>" is
misspelled by doing a dummy parsing before the main body of "gc"
that is time consuming even begins.  Otherwise, we'd spend time to
pack objects and then later have "git prune" first notice the error.
Aborting "gc" in the middle that way is not harmful but is ugly and
can be avoided.

Helped-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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