Improved it a bit, so it says what it's doing, and defaults to master when there's no branch-specific setting

post-commit

#! /bin/zsh

branch_name=${$(git symbolic-ref HEAD)#refs/heads/}

function switch {
  echo "Switching to database.yml.branch.${1}"
  cp config/database.yml config/database.yml.branch.bak
  cp config/database.yml.branch.${1} config/database.yml
}

if [ -f config/database.yml.branch.${branch_name} ]; then
    echo "Switching to database.yml.branch.${branch_name}"
    switch ${branch_name}
else
    switch 'master'    
fi