builtin/reflog: improve grouping of subcommands

builtin/reflog: improve grouping of subcommands

The way subcommands of git-reflog(1) are laid out does not make any
immediate sense. Reorder them such that read-only subcommands precede
writing commands for a bit more structure.

Furthermore, move the "expire" subcommand last. This prepares for a
subsequent change where we are about to introduce a new "write" command
to append reflog entries. Like this, the writing subcommands are ordered
such that those affecting a single reflog come before those spanning
across all reflogs.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>