gc: handle & check gc.reflogExpire config

gc: handle & check gc.reflogExpire config

Don't redundantly run "git reflog expire --all" when gc.reflogExpire
and gc.reflogExpireUnreachable are set to "never", and die immediately
if those configuration valuer are bad.

As an earlier "assert lack of early exit" change to the tests for "git
reflog expire" shows, an early check of gc.reflogExpire{Unreachable,}
isn't wanted in general for "git reflog expire", but it makes sense
for "gc" because:

 1) Similarly to 8ab5aa4bd8 ("parseopt: handle malformed --expire
    arguments more nicely", 2018-04-21) we'll now die early if the
    config variables are set to invalid values.

    We run "pack-refs" before "reflog expire", which can take a while,
    only to then die on an invalid gc.reflogExpire{Unreachable,}
    configuration.

 2) Not invoking the command at all means it won't show up in trace
    output, which makes what's going on more obvious when the two are
    set to "never".

 3) As a later change documents we lock the refs when looping over the
    refs to expire, even in cases where we end up doing nothing due to
    this config.

    For the reasons noted in the earlier "assert lack of early exit"
    change I don't think it's worth it to bend over backwards in "git
    reflog expire" itself to carefully detect if we'll really do
    nothing given the combination of all its possible options and skip
    that locking, but that's easy to detect here in "gc" where we'll
    only run "reflog expire" in a relatively simple mode.

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