builtin/reflog: introduce subcommand to list reflogs

While the git-reflog(1) command has subcommands to show reflog entries or check for reflog existence, it does not have any subcommands that would allow the user to enumerate all existing reflogs. This makes it quite hard to discover which reflogs a repository has. While this can be worked around with the "files" backend by enumerating files in the ".git/logs" directory, users of the "reftable" backend don't enjoy such a luxury.

Introduce a new subcommand git reflog list that lists all reflogs the repository knows of to fill this gap.

Closes #269 (closed).