reflog: actually default to subcommand 'show'

reflog: actually default to subcommand 'show'

The reflog manpage says:

	git reflog [show] [log-options] [<ref>]

the subcommand 'show' is the default "in the absence of any
subcommands". Currently this is only true if the user provided either
at least one option or no additional argument at all. For example:

	git reflog master

won't work. Change this by actually calling cmd_log_reflog in
absence of any subcommand.

Signed-off-by: Michael Schubert <redacted>
Signed-off-by: Junio C Hamano <redacted>