utils/git: various improvement

* Use `ENV/scm/git` to check whether git is installed.
* Use `brew install git` to enforce pre-install checks such as sudo
  check.

Library/Homebrew/utils/git.rb

@@ -1,27 +1,20 @@
 module Utils
   def self.git_available?
     return @git if instance_variable_defined?(:@git)
-    # check git in original path in case it's the wrapper script of Library/ENV/scm
+    @git = quiet_system HOMEBREW_ENV_PATH/"scm/git", "--version"
-    git = which("git", ORIGINAL_PATHS.join(File::PATH_SEPARATOR))
-    # git isn't installed by older Xcodes
-    return @git = false if git.nil?
-    # /usr/bin/git is a popup stub when Xcode/CLT aren't installed, so bail out
-    return @git = false if git == "/usr/bin/git" && !OS::Mac.has_apple_developer_tools?
-    @git = true
   end
 
   def self.ensure_git_installed!
     return if git_available?
 
-    require "cmd/install"
     begin
       oh1 "Installing git"
-      Homebrew.perform_preinstall_checks
+      safe_system HOMEBREW_BREW_FILE, "install", "git"
-      Homebrew.install_formula(Formulary.factory("git"))
     rescue
       raise "Git is unavailable"
     end
 
+    clear_git_available_cache
     raise "Git is unavailable" unless git_available?
   end