This prepares a place to collect reflog management subcommands,
and implements "expire" action.

	$ git reflog expire --dry-run \
		--expire=4.weeks \
		--expire-unreachable=1.week \
		refs/heads/master

The expiration uses two timestamps: --expire and --expire-unreachable.
Entries older than expire time (defaults to 90 days), and entries older
than expire-unreachable time (defaults to 30 days) and records a commit
that has been rewound and made unreachable from the current tip of the
ref are removed from the reflog.

The parameter handling is still rough, but I think the
core logic for expiration is already sound.
Signed-off-by: NJunio C Hamano <junkio@cox.net>