(maint) Add helper functions for release lead work

This adds two rake tasks that help with release lead duties.

- Platform differences:
  `release_lead:platform_diff`: Given two puppet-agent SHAs or tags,
  this task reports which platforms have been added and retired between
  the first and the second. Example execution:
```
  bundle exec rake release_lead:platform_diff[tag,SHA]
```

- Checking components:
  `release_lead:check_components`: Outputs all component statuses at a
  given SHA or tag by checking out that git reference, cloning all
  linked components, and running `git describe` on each, reporting the
  tag and any commits since the last tag. Example execution:
```
  bundle exec rake release_lead:check_components[SHA]
```

Rakefile

@@ -1,5 +1,7 @@
 require 'packaging'
 
+load './ext/release-lead.rake'
+
 Pkg::Util::RakeUtils.load_packaging_tasks
 
 namespace :package do

ext/release-lead.rake

@@ -0,0 +1,118 @@
+require 'json'
+require 'optparse'
+
+namespace :release_lead do
+  desc "Find platforms added and removed between releases"
+  task :platform_diff, [:from, :to] do |t, args|
+    abort ('`from` argument is required') unless args.from
+    abort ('`to` argument is required') unless args.to
+    puts `git diff --summary #{args.from}..#{args.to} configs/platforms`.
+        gsub('configs/platforms/', '').gsub(/mode \d+ /, '').
+        gsub('delete', 'Retired').gsub('create', 'Added').
+        gsub('rename', 'Renamed')
+  end
+
+  # Calculate name of repository from the .git URL.
+  def url_to_component_name(url)
+    url.split('/').last.split('.').first
+  end
+
+  # Calculate name of repository from the name of the component's .json file.
+  def json_name_to_component_name(json_filename)
+    json_filename.split('/').last.split('.').first
+  end
+
+  # Input a git repository, a SHA or tag to check out, and a place where it can be cloned.
+  # Output the resulting `git describe` for that reference.
+  def git_describe_repo(name, url, sha_or_tag, where_to_clone, show_extra_commits)
+    Dir.chdir(where_to_clone) do
+      puts "Cloning #{name}..."
+      `git clone #{url} 2> /dev/null`
+      # `git clone --local '../../#{name}' 2> /dev/null`
+      # 0 for successful clone, 128 for already exists.
+      unless [0, 128].include? $?.exitstatus
+        next 'Could not find remote repository'
+      end
+      Dir.chdir(name) do
+        `git fetch 2> /dev/null`
+        `git checkout #{sha_or_tag} 2> /dev/null`
+        unless $?.exitstatus == 0
+          puts "Could not find reference #{sha_or_tag}"
+          exit
+        end
+        `git describe --tags`.strip +
+            if show_extra_commits
+              lines = `git log \`git describe --tags --abbrev=0\`..HEAD --oneline`.split("\n")
+              lines.map do |value|
+                if value.include?('[no-promote]')
+                  # Output these in red.
+                  "\e[31m#{value}\e[0m"
+                else
+                  value
+                end
+              end.join("\n  ")
+            else
+              ''
+            end
+      end
+    end
+  end
+
+  def check_components(args, show_extra_commits = false)
+    result = {}
+    abort('Error: puppet_agent_branch argument is required') unless args.puppet_agent_branch
+
+    where_to_clone = File.join(File.dirname(__FILE__), 'pkg')
+    Dir.mkdir(where_to_clone) unless Dir.exists?(where_to_clone)
+
+    # Let's ensure puppet-agent is on the right branch.
+    `git fetch`
+    begin
+      `git checkout #{args.puppet_agent_branch} 2> /dev/null`
+
+      Dir.glob("./configs/components/*.json").each do |component|
+        json = JSON.parse(File.read(component))
+        url = json['url']
+        if url.nil?
+          name = json_name_to_component_name(component)
+          result[name] = 'No URL provided'
+          next
+        end
+        name = url_to_component_name(url)
+        ref = json['ref']
+        sha_or_tag = ref =~ /^refs\/tags/ ? ref.gsub('refs/tags/', '') : ref
+        result[name] = git_describe_repo(name, url, sha_or_tag, where_to_clone, show_extra_commits)
+      end
+
+      max_length = result.keys.map(&:length).max
+      puts "\n** Latest versions in #{args.puppet_agent_branch}:\n"
+      result.map do |name, value|
+        puts name.ljust(max_length + 1) + ': ' + value
+      end
+    ensure
+      `git checkout @{-1} 2> /dev/null`
+    end
+  end
+
+  # This task performs the following for each repository:
+  # - Looks up each component
+  # - Clones the component into the 'pkg' directory, if not done already
+  # - Checks out the git reference that's in the component's .json.
+  # - Outputs the `git describe` for that reference.
+  # This is useful for determining tag versions for an upcoming release.
+  desc "Output `git describe` for each component's git branch"
+  task :check_components_diff, [:puppet_agent_branch] do |t, args|
+    check_components(args, true)
+  end
+
+  # This task performs the following for each repository:
+  # - Looks up each component
+  # - Clones the component into the 'pkg' directory, if not done already
+  # - Checks out the git reference that's in the component's .json.
+  # - Outputs the `git describe` for that reference.
+  # This is useful for determining tag versions for an upcoming release.
+  desc "Output `git describe` for each component's git branch"
+  task :check_components, [:puppet_agent_branch] do |t, args|
+    check_components(args, false)
+  end
+end