Dpkg::Changelog::Parse: Set a default filename in changelog_parse()

Regression introduced in commit 43556e0a9f216d59b4bc1a0f4e6e10e9afea3f7d,
during a botched rebase.

scripts/Dpkg/Changelog/Parse.pm

@@ -256,6 +256,7 @@ sub changelog_parse {
     my (%options) = @_;
 
     $options{forceplugin} //= 0;
+    $options{file} //= 'debian/changelog';
     $options{changelogformat} //= _changelog_detect_format($options{file});
 
     if (not $options{forceplugin} and