reflog: implement subcommand to drop reflogs

This patch series adds a new 'drop' subcommand to git-reflog that allows users to delete the entire reflog for a specified reference. Include a '--all' flag to enable dropping all reflogs in a repository.

This was discussed when I sent in a patch to add '--no-reflog' option to 'git refs migrate' 1.

The first patch is a small cleanup which ensures that 'builtin/reflog.c' no longer uses global variables. The second patch add the required changes.

--- b4-submit-tracking ---

This section is used internally by b4 prep for tracking purposes.

{ "series": { "revision": 1, "change-id": "20250306-493-add-command-to-purge-reflog-entries-bd22547ad34a", "prefixes": [] } }