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>

pack-refs.h

@@ -0,0 +1,23 @@
+#ifndef PACK_REFS_H
+#define PACK_REFS_H
+
+struct repository;
+
+/*
+ * Shared usage string for options common to git-pack-refs(1)
+ * and git-refs-optimize(1). The command-specific part (e.g., "git refs optimize ")
+ * must be prepended by the caller.
+ */
+#define PACK_REFS_OPTS \
+	"[--all] [--no-prune] [--auto] [--include <pattern>] [--exclude <pattern>]"
+
+/*
+ * The core logic for pack-refs and its clones.
+ */
+int pack_refs_core(int argc,
+		   const char **argv,
+		   const char *prefix,
+		   struct repository *repo,
+		   const char * const *usage_opts);
+
+#endif /* PACK_REFS_H */