manually push option arguments to array instead of using s@

By mixing s@ for --$foo-hook options and manual pushing in --hook-dir,
it can happen that options get lost. Consider the following test:

use Getopt::Long;
my $arr = [];
GetOptions(
    'A=s@' => \$arr,
    'B=s' => sub { push @{$arr}, $_[1]; }
);
foreach my $hook (@{$arr}) { print "hook: $hook\n"; }

This works fine:

    perl test.pl --A=a1 --B=b1 --A=a2 --B=b2
    hook: a1
    hook: b1
    hook: a2
    hook: b2

This misses b1:

    perl test.pl --B=b1 --A=a2 --B=b2
    hook: a2
    hook: b2

mmdebstrap

@@ -4284,11 +4284,19 @@ sub main() {
           sub { push @{ $options->{noop} }, 'no-merged-usr'; },
         'force-check-gpg' =>
           sub { push @{ $options->{noop} }, 'force-check-gpg'; },
-        'setup-hook=s@'     => \$options->{setup_hook},
-        'extract-hook=s@'   => \$options->{extract_hook},
-        'essential-hook=s@' => \$options->{essential_hook},
-        'customize-hook=s@' => \$options->{customize_hook},
-        'hook-directory=s'  => sub {
+        'setup-hook=s' => sub {
+            push @{ $options->{setup_hook} }, $_[1];
+        },
+        'extract-hook=s' => sub {
+            push @{ $options->{extract_hook} }, $_[1];
+        },
+        'essential-hook=s' => sub {
+            push @{ $options->{essential_hook} }, $_[1];
+        },
+        'customize-hook=s' => sub {
+            push @{ $options->{customize_hook} }, $_[1];
+        },
+        'hook-directory=s' => sub {
             my ($opt_name, $opt_value) = @_;
             if (!-e $opt_value) {
                 error "hook directory \"$opt_value\" does not exist";