send-pack: properly use parse_options() API for usage string

send-pack: properly use parse_options() API for usage string

When "send-pack" was changed to use the parse_options() API in
068c77a5189 (builtin/send-pack.c: use parse_options API, 2015-08-19)
it was made to use one very long line, instead it should split them up
with newlines.

Furthermore we were including an inline explanation that you couldn't
combine "--all" and "<ref>", but unlike in the "blame" case this was
not preceded by an empty string.

Let's instead show that --all and <ref> can't be combined in the the
usual language of the usage syntax instead. We can make it clear that
one of the two options "--foo" and "--bar" is mandatory, but that the
two are mutually exclusive by referring to them as "( --foo | --bar
)".

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