builtin/pack-refs: convert to use the generic refs_optimize() API
The `git pack-refs` command behaves generically, triggering a pack for the 'files' backend and a compaction for the 'reftable' backend. However, the name of the command and its corresponding API is conceptually tied to the 'files' backend implementation. To create a cleaner, more generic interface, refactor `git pack-refs` to use the new `refs_optimize()` API. "Optimize" is a better semantic term for this generic action. This change allows `git pack-refs` to act as a backend-agnostic frontend for reference optimization, and paves the way for the new `git refs optimize` command to do the same. 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>