builtin/refs: add '--no-reflog' flag to drop reflogs

The "git refs migrate" subcommand converts the backend used for ref
storage. It always migrates reflog data as well as refs. Introduce an
option to exclude reflogs from migration, allowing them to be discarded
when they are unnecessary.

This is particularly useful in server-side repositories, where reflogs
are typically not expected. However, some repositories may still have
them due to historical reasons, such as bugs, misconfigurations, or
administrative decisions to enable reflogs for debugging. In such
repositories, it would be optimal to drop reflogs during the migration.

To address this, introduce the '--no-reflog' flag, which prevents reflog
migration. When this flag is used, reflogs from the original reference
backend are migrated. Since only the new reference backend remains in
the repository, all previous reflogs are permanently discarded.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -1157,8 +1157,11 @@ int is_pseudo_ref(const char *refname);
  *   - REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN: perform a dry-run migration
  *     without touching the main repository. The result will be written into a
  *     temporary ref storage directory.
+ *
+ *   - REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG: skip migration of reflogs.
  */
-#define REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN (1 << 0)
+#define REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN      (1 << 0)
+#define REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG (1 << 1)
 
 /*
  * Migrate the ref storage format used by the repository to the