Simplify helper script through the power of set operators

tools/build/missing-commits.pl6

@@ -5,23 +5,21 @@ sub commit($line) { $line.starts-with('commit ') ?? $line.substr(7,8) !! Nil }
 # The commit that did the last VERSION bump
 my $version-bump-commit = commit( qx/git log -- VERSION/.lines.head );
 
-# Make a SetHash with all of the commits since the last VERSION bump
-my %commits is SetHash = qx/git log/.lines.map( {
+# Make a Set with all of the commits since the last VERSION bump minus
+# all the commits that have been documented in ChangeLog since the last
+# bump.  Note that we assume that an empty line indicates the boundary
+# between releases.
+my $missing = qx/git log/.lines.map( {
     if commit($_) -> \commit {
         last if commit eq $version-bump-commit;
         commit
     }
-} );
-
-# Remove the commits that have been documented in ChangeLog since bump
-# Note that we assume here that an empty line indicates boundary between
-# releases.
-%commits{ "docs/ChangeLog".IO.lines.map( {
+} ) (-) "docs/ChangeLog".IO.lines.map: {
     $_
       ?? |.comb( / <?after \[> ( <[ 0..9 a..f A..F ]> ** 8 ) <?before \]> / )
       !! last
-} ) }:delete;
+}
 
 # Let the world know the result
-say "%commits.elems() commits not yet documented in ChangeLog:";
-%commits.keys.sort.batch(9).map: *.Str.say;
+say "$missing.elems() commits not yet documented in ChangeLog:";
+$missing.keys.sort.batch(9).map: *.Str.say;