Add the `-0' option so that a flycheck checker can be written. These
don't seem to like it when the checker exits with a non-zero status,
whereas for the purposes of being a `commit-msg' hook for git it is
important that it does exit with a non-zero status to stop git from
committing.
Check the file whose name has been specified on the command line for
errors, instead of `stdin'. Git passes the file name of the file
containing the commit message instead of passing the commit message to
`stdin'.
This was supposed to be committed in the first commit. but somehow it
got lost.
