#!/usr/bin/perl -w
use strict;

open( IN, "git log --merges --format=\"%P %s\" |" ) || die;

my %branches;

while (<IN>) {
    /^([0-9a-f]+)\s([0-9a-f]+)\sMerge branch '(.+?)'/ || next;
    $branches{$3} = $2 unless defined $branches{$3};
}

close(IN);

print $_. " " . $branches{$_} . "\n" foreach sort keys %branches;