Add -h and -0 command-line options

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.

commit-check

@@ -24,6 +24,33 @@ first command-line argument. The style that's checked is based on
 tpope's L<A Note About Git Commit
 Messages|http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html>.
 
+=head2 Command-line options
+
+There are 2 command line options available for use with this program:
+
+=over
+
+=item -0
+
+Don't return a non-zero status when errors have been found. For use as
+a git C<commit-msg> hook it is important that it does exit with a
+non-zero status. But if you are using it for other tools, such as for
+a flycheck checker, exiting with a non-zero status might confuse the
+tools calling it.
+
+=item -h
+
+Show a little help text that describes the command line options and
+usage.
+
+=back
+
+When no arguments are passed (not even the file name) the help text is
+also shown and the exit status is non-0 (no matter if C<-0> has been
+given).
+
+=head2 Errors
+
 Currently there are 4 style errors that are checked for.
 
 =over
@@ -51,19 +78,21 @@ No line should be longer than 72 characters.
 All comments are skipped and so is all whitespace at the beginning of
 the file.
 
-If any of the error conditions are encountered the program will exit
+If any of the error conditions are encountered, and the C<-0> option
-with a non-C<0> status. This has the effect of stopping git from
+has not been given, the program will exit with a non-C<0> status. This
-committing the message when this program is used as a C<commit-msg>
+has the effect of stopping git from committing the message when this
-hook. Each error condition encountered also prints a message to the
+program is used as a C<commit-msg> hook. Each error condition
-standard error stream.
+encountered also prints a message to the standard error stream.
 
 =cut
 
 use strict;
 use warnings;
+use Getopt::Std;
 
 my $lineno = 0;
 my $status = 0;
+my %arguments = ();
 
 sub err {
     my ($msg) = @_;
@@ -72,6 +101,21 @@ sub err {
     $status = 1;
 }
 
+sub usage {
+    my ($status) = @_;
+    print "Usage: commit-check [-0|-h] <file>\n"
+        . "\n"
+        . "Accepted arguments:\n"
+        . "  -0   Always return 0 exit-status, regardless of errors found.\n"
+        . "  -h   Show this help text.\n";
+    exit $status;
+}
+
+getopts("0h", \%arguments);
+
+usage 0 if $arguments{h};
+usage 1 if !$ARGV[0];
+
 open(my $commitfile, "<", $ARGV[0]) or die "Couldn't open $ARGV[0]";
 
 while (<$commitfile>) {
@@ -90,4 +134,5 @@ while (<$commitfile>) {
 }
 
 close $commitfile;
-exit $status;
+exit $status if !$arguments{0};
+exit 0;