Auto-labeling Git commits with Pivotal Tracker IDs found in the current branch name. This script is meant to be used as a Git `commit-msg` hook. It expects a single argument, the location of the temporary file with the commit message the user entered, and it outputs that same message with maybe some extra labels added to it.

auto_reference_pt_ids.rb

#!/usr/bin/env ruby
# ## Introduction
#
# Auto-labeling Git commits with issues tracker IDs found in the current
# branch name. This script is meant to be used as a Git commit-msg hook. It
# expects a single argument, the location of the temporary file with the commit
# message the user entered, and it outputs that same message with maybe some
# extra labels added to it.
#
# ## Example
#
# For example, when on branch `feature_123456_my_new_feature`, it will transform a
# commit message like this:
#
#     Implement new feature
#
# Into something like this:
#
#     Implement new feature
#
#     [#123456]
#
# When your commit message already includes a reference to the user story ID
# this script would want to add, it does not add it again. Also, when you
# include the special `NOREF` string somewhere in the commit message, that will
# be stripped out, but no other changes will be made.
#
# ## Supported formats
#
# The following formats are currently supported:
#
# * Pivotal Tracker IDs: 7-9 digits
# * Jira issue IDs: Two or more capitals, dash, one or more digits
#
# You can add more formats by subclassing the `Issue` class.
#
# ## Testing
#
# To run this script's tests, simply run it with no arguments:
#
#     $ ./auto_reference_pt_ids
#
# This will cause it to run its own unit tests (requires Rubygems and Rspec).
#
# ## Installation
#
# To install, copy it into `path/to/project/.git/hooks/commit-msg` and make it
# executable.
#
# Author: Arjan van der Gaag
# URL:    http://arjanvandergaag.nl
class Issue
  attr_reader :branch_name
  private :branch_name

  @subclasses = []

  def self.inherited(subclass)
    @subclasses << subclass
  end

  def self.matching(branch_name)
    @subclasses
      .map  { |subclass| subclass.new(branch_name) }
      .find { |issue| issue.matches? } || NullIssue.new
  end

  def initialize(branch_name)
    @branch_name = branch_name
  end

  def id
    branch_name[pattern, 1]
  end

  def id?
    true
  end

  def matches?
    pattern === branch_name
  end

  def pattern
    //
  end
end

class NullIssue
  def matches?
    true
  end

  def id?
    false
  end

  def format
    ''
  end
end

class JiraIssue < Issue
  def pattern
    /([A-Z]{2,}[\-_]\d+)/
  end

  def format
    "\n\n%s" % id.sub('_', '-')
  end
end

class PivotalTrackerIssue < Issue
  def pattern
    /(\d{7,9})/
  end

  def format
    "\n\n[#%s]" % id
  end
end

class Labeler
  OUTPUT_FORMAT = "\n\n[#%s]"

  ID_IN_BRANCH_FORMAT = /(\d{7,9}|[A-Z]{2,}[\-_]\d+)/

  ESCAPE = 'NOREF'

  attr_reader :original_message

  def initialize(original_message)
    @original_message = original_message
  end

  def filtered_message
    if requires_update?
      original_message + formatted_id_from_branch_name
    elsif contains_escape?
      original_message.sub(ESCAPE, '').strip
    else
      original_message
    end
  end

  def branch_name
    @branch_name ||= `git branch --no-color 2>/dev/null`[/^\* (\w+)$/, 1]
  end

  private

  def requires_update?
    !contains_escape? &&
      branch_name_contains_id? &&
      !original_message_already_contains_reference?
  end

  def contains_escape?
    original_message.include?(ESCAPE)
  end

  def formatted_id_from_branch_name
    issue.format
  end

  def issue
    @issue ||= Issue.matching(branch_name)
  end

  def original_message_already_contains_reference?
    original_message.include?(formatted_id_from_branch_name.strip)
  end

  def branch_name_contains_id?
    branch_name && issue.id?
  end
end

if ARGV.any?
  commit_message_filename = ARGV[0]
  labeler = Labeler.new(File.read(commit_message_filename))
  File.open(commit_message_filename, 'w') do |f|
    f.write(labeler.filtered_message)
  end
else
  require 'rubygems'
  require 'rspec/autorun'

  describe Labeler do
    let(:message) { "Add new feature.\n\nDetailed description." }
    subject       { described_class.new(message) }

    before do
      allow(subject).to receive(:`).with('git branch --no-color 2>/dev/null').and_return(branch_list)
    end

    context 'on a regular branch' do
      let(:branch_list)      { "* new_feature\n  other_branch" }
      its(:branch_name)      { should eql('new_feature') }
      its(:filtered_message) { should eql(message) }
    end

    context 'on a branch with a JIRA issue ID' do
      let(:branch_list) { "  other_branch\n* JIR_136_timezones" }
      its(:branch_name) { should eql('JIR_136_timezones') }

      context 'when using a simple message' do
        its(:filtered_message) { should eql("Add new feature.\n\nDetailed description.\n\nJIR-136") }
      end

      context 'when it already mentions the ID' do
        let(:message)          { "JIR-136: Add new feature.\n\nDetailed description." }
        its(:filtered_message) { should eql(message) }
      end

      context 'when it contains the special NOREF comment' do
        let(:message)          { "Add new feature.\n\nDetailed description. NOREF" }
        its(:filtered_message) { should eql("Add new feature.\n\nDetailed description." ) }
      end
    end

    context 'on a branch with a Pivotal Tracker ID' do
      let(:branch_list) { "  other_branch\n* 12345678_new_feature" }
      its(:branch_name) { should eql('12345678_new_feature') }

      context 'when using a simple message' do
        its(:filtered_message) { should eql("Add new feature.\n\nDetailed description.\n\n[#12345678]") }
      end

      context 'when it already mentions the ID' do
        let(:message)          { "Add new feature.\n\nDetailed description. [#12345678]" }
        its(:filtered_message) { should eql(message) }
      end

      context 'when it contains the special NOREF comment' do
        let(:message)          { "Add new feature.\n\nDetailed description. NOREF" }
        its(:filtered_message) { should eql("Add new feature.\n\nDetailed description." ) }
      end
    end

    context 'not on any branch' do
      let(:branch_list)      { '' }
      its(:branch_name)      { should be_nil }
      its(:filtered_message) { should eql(message) }
    end
  end
end

Why only the ID and not include a link to the story as well?

This is only about using Pivotal Tracker's SCM integration. I've never thought of including links before. It's not as if URLs in git-log output would be clickable, so they'd save you the copy/pasting…

Now, putting the link to a user story in a pull request description, that would be useful, no?