Add pivotal tracker story number to git commit messages.

gistfile1.rb

#!/usr/bin/env ruby

# put this in .git/hooks/prepare-commit-msg

message_file = ARGV[0]
message = File.read(message_file)

pivotal_story_number = `git rev-parse --abbrev-ref HEAD | grep --only-matching '[0-9]*$'`.strip

# do not add anything if the branch doesn't have a story number or the message already contains one.
unless pivotal_story_number.empty? || message =~ /^\[#\d+\]/
  message = "[##{pivotal_story_number}]\n#{message}"
end

File.open(message_file, 'w') { |f| f.write(message) }