Add git-churn script

After you commit a bug fix to a feature branch, find out if the code
you changed to fix the bug is in files that change often:

    git-churn

If the buggy code changes often, find smells and refactor them. Separate
the parts that change often from the parts that don't.

Conversely, avoid refactoring areas with low churn. Refactoring changes
code, and with each change, you risk introducing new bugs.

Example:

     [feature-branch][~/dev/rails-app] churn
     2 app/controllers/accepts_controller.rb
     2 config/database.yml
     4 app/helpers/application_helper.rb
     4 config/application.rb
     9 spec/models/user_spec.rb
     9 spec/spec_helper.rb
    12 spec/requests/dashboard_spec.rb
    13 spec/models/feedback_spec.rb
    19 app/models/feedback.rb
    21 app/models/user.rb

.gitignore

@@ -1 +1,2 @@
 vim/.netrwhist
+!bin

bin/git-churn

@@ -0,0 +1,64 @@
+#!/usr/bin/ruby
+#
+# Credits: Corey Haines, Gary Bernhardt, Dan Croak
+#
+# Put this on your $PATH (~/bin is recommended).
+#
+# Show churn for the files changed in the branch:
+#
+#   $ git-churn
+#
+
+class GitChurn
+  attr_accessor :all_files_with_churn_count, :changed_files, :result
+
+  def calculate
+    get_all_files_with_churn_count
+    get_changed_files
+    filter_into_result
+    print_result
+  end
+
+  private
+
+  def get_all_files_with_churn_count
+    self.all_files_with_churn_count =
+      `git log --all #{format_logs} | #{remove_blank_lines} | #{sort_by_ascending_churn_count}`.split("\n")
+  end
+
+  def format_logs
+    "--name-only --format='format:'"
+  end
+
+  def remove_blank_lines
+    "grep -v '^$'"
+  end
+
+  def sort_by_ascending_churn_count
+    'sort | uniq -c | sort'
+  end
+
+  def get_changed_files
+    self.changed_files =
+      `git log origin/master..HEAD #{format_logs} | #{remove_blank_lines} | #{remove_duplicates}`.split("\n")
+  end
+
+  def remove_duplicates
+    'sort | uniq'
+  end
+
+  def filter_into_result
+    self.result = all_files_with_churn_count.select { |file| file_changed?(file) }.join("\n")
+  end
+
+  def file_changed?(file)
+    changed_files.any? { |changed_file| file.include?(changed_file) }
+  end
+
+  def print_result
+    puts result
+  end
+end
+
+git_churn = GitChurn.new
+git_churn.calculate