Allow checking a single commit more easily

The "check commits" subcommand allows you to check a range
of commits. If the second commit argument is missing, it
defaults to HEAD. So in order to check a single commit, you
have to pass the arguments "check commits $commit~1 $commit".
For the single commit argument case, we can do this on it's
own so that the user only need to pass "check commit $commit".

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>