builtin/refs: add '--skip-reflog' flag to bypass reflog migration

The 'git-refs(1)' migrate subcommand, which transfers repositories between reference backends, currently migrates reflogs by default as of In 246cebe3 (refs: add support for migrating reflogs, 2024-12-16). While this behavior is desirable for most client-side repositories, server-side repositories typically don't use reflogs and the migration of these entries is unnecessary overhead.

Add a '--skip-reflog' flag to the migrate subcommand to make reflog migration optional. This is particularly useful for server-side migrations where reflogs are not needed, improving migration performance in these scenarios.

Closes: #477 (closed)