doc: pack-refs: factor out common options

doc: pack-refs: factor out common options

In preparation for adding documentation for `git refs optimize`, factor
out the common options from the `git-pack-refs` man page into a
shareable file `pack-refs-options.adoc` and update `git-pack-refs.adoc`
to use an `include::` macro.

This change is a pure refactoring and results in no change to the final
rendered documentation for `pack-refs`.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>