builtin/pack-refs: factor out core logic into a shared library

The implementation of `git pack-refs` is monolithic within
`cmd_pack_refs()`, making it impossible to share its logic with other
commands. To enable code reuse for the upcoming `git refs optimize`
subcommand, refactor the core logic into a shared helper function.

Split the original `builtin/pack-refs.c` file into two parts:

- A new shared library file, `pack-refs.c`, which contains the
  core option parsing and packing logic in a new `pack_refs_core()`
  helper function.

- The original `builtin/pack-refs.c`, which is now a thin wrapper
  responsible only for defining the `git pack-refs` command and
  calling the shared helper.

A new `pack-refs.h` header is also introduced to define the public
interface for this shared logic.

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>

meson.build

@@ -407,6 +407,7 @@ libgit_sources = [
   'pack-check.c',
   'pack-mtimes.c',
   'pack-objects.c',
+  'pack-refs.c',
   'pack-revindex.c',
   'pack-write.c',
   'packfile.c',