doctor: use Utils.git_available? instead of git?

Library/Homebrew/cmd/doctor.rb

@@ -59,14 +59,6 @@ class Checks
   def inject_file_list(list, str)
     list.inject(str) { |s, f| s << "    #{f}\n" }
   end
-
-  # Git will always be on PATH because of the wrapper script in
-  # Library/ENV/scm, so we check if there is a *real*
-  # git here to avoid multiple warnings.
-  def git?
-    return @git if instance_variable_defined?(:@git)
-    @git = system "git --version >/dev/null 2>&1"
-  end
   ############# END HELPERS
 
   # Sorry for the lack of an indent here, the diff would have been unreadable.
@@ -884,7 +876,7 @@ class Checks
   end
 
   def check_for_git
-    if git?
+    if Utils.git_available?
       __check_git_version
     else <<-EOS.undent
     Git could not be found in your PATH.
@@ -896,7 +888,7 @@ class Checks
   end
 
   def check_git_newline_settings
-    return unless git?
+    return unless Utils.git_available?
 
     autocrlf = `git config --get core.autocrlf`.chomp
 
@@ -914,7 +906,7 @@ class Checks
   end
 
   def check_git_origin
-    return unless git? && (HOMEBREW_REPOSITORY/".git").exist?
+    return if !Utils.git_available? || !(HOMEBREW_REPOSITORY/".git").exist?
 
     origin = Homebrew.git_origin
 
@@ -1026,7 +1018,7 @@ class Checks
   end
 
   def check_git_status
-    return unless git?
+    return unless Utils.git_available?
     HOMEBREW_REPOSITORY.cd do
       unless `git status --untracked-files=all --porcelain -- Library/Homebrew/ 2>/dev/null`.chomp.empty?
         <<-EOS.undent_________________________________________________________72
@@ -1120,7 +1112,7 @@ class Checks
   end
 
   def check_for_outdated_homebrew
-    return unless git?
+    return unless Utils.git_available?
     HOMEBREW_REPOSITORY.cd do
       if File.directory? ".git"
         local = `git rev-parse -q --verify refs/remotes/origin/master`.chomp

Library/Homebrew/utils/git.rb

@@ -1,7 +1,8 @@
 module Utils
   def self.git_available?
     return @git if instance_variable_defined?(:@git)
-    git = which("git")
+    # check git in original path in case it's the wrapper script of Library/ENV/scm
+    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