add git utils

Two methods:
* `Utils.git_available?` checks whether git is installed.
* `Utils.ensure_git_installed!` installs git for users who don't install
  Xcode or CLT.

Library/Homebrew/utils.rb

@@ -5,6 +5,7 @@ require "utils/json"
 require "utils/inreplace"
 require "utils/popen"
 require "utils/fork"
+require "utils/git"
 require "open-uri"
 
 class Tty

Library/Homebrew/utils/git.rb

+module Utils
+  def self.git_available?
+    git = which("git")
+    # git isn't installed by older Xcodes
+    return false if git.nil?
+    # /usr/bin/git is a popup stub when Xcode/CLT aren't installed, so bail out
+    return false if git == "/usr/bin/git" && !OS::Mac.has_apple_developer_tools?
+    true
+  end
+
+  def self.ensure_git_installed!
+    return if git_available?
+
+    require "cmd/install"
+    begin
+      oh1 "Installing git"
+      Homebrew.perform_preinstall_checks
+      Homebrew.install_formula(Formulary.factory("git"))
+    rescue
+      raise "Git is unavailable"
+    end
+  end
+end