Fix usage of checking a commit range

When checking a commit range, the subcommand "commits" as well
as the first parameter "since" are marked as being optional in
the usage section but actually they aren't. Change the usage to
make it clear which arguments are optional and which are not.

These arguments were probably optional because the "check" command
can be run without any arguments to check "everything". Add a new
entry in the usage section for this usage of "check".

While at there, fix style of optional arguments in the usage
section according to git.git.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
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>