Add GitRepositoryExtension

Add an extension for accessing Git-related meta data that can be mixed
in into a Pathname object (e.g. `HOMBREW_REPOSITORY` or the path of a
`Tap` instance). The goal here is to eliminate code duplication.

Library/Homebrew/extend/git_repository.rb

+require "utils/git"
+require "utils/popen"
+
+module GitRepositoryExtension
+  def git?
+    join(".git").exist?
+  end
+
+  def git_origin
+    return unless git? && Utils.git_available?
+    cd do
+      Utils.popen_read("git", "config", "--get", "remote.origin.url").chuzzle
+    end
+  end
+
+  def git_head
+    return unless git? && Utils.git_available?
+    cd do
+      Utils.popen_read("git", "rev-parse", "--verify", "-q", "HEAD").chuzzle
+    end
+  end
+
+  def git_short_head
+    return unless git? && Utils.git_available?
+    cd do
+      Utils.popen_read(
+        "git", "rev-parse", "--short=4", "--verify", "-q", "HEAD"
+      ).chuzzle
+    end
+  end
+
+  def git_last_commit
+    return unless git? && Utils.git_available?
+    cd do
+      Utils.popen_read("git", "show", "-s", "--format=%cr", "HEAD").chuzzle
+    end
+  end
+
+  def git_last_commit_date
+    return unless git? && Utils.git_available?
+    cd do
+      Utils.popen_read(
+        "git", "show", "-s", "--format=%cd", "--date=short", "HEAD"
+      ).chuzzle
+    end
+  end
+end

Library/Homebrew/global.rb

 require "extend/module"
 require "extend/fileutils"
 require "extend/pathname"
+require "extend/git_repository"
 require "extend/ARGV"
 require "extend/string"
 require "extend/enumerable"