doctor: check git version

Checking out over HTTP/HTTPS from GitHub requires git 1.6.6, as GitHub
only allows Smart HTTP transport.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/doctor.rb

@@ -776,6 +776,28 @@ def check_for_leopard_ssl
   end
 end
 
+def check_git_version
+  # see https://github.com/blog/642-smart-http-support
+  return unless system "/usr/bin/which -s git"
+  `git --version`.chomp =~ /git version (\d)\.(\d)\.(\d)/
+
+  if $2.to_i > 6
+    return
+  elsif $2.to_i == 6 and $3.to_i == 6
+    return
+  else
+    puts <<-EOS.undent
+      An outdated version of Git was detected in your PATH.
+
+      Git 1.6.6 or newer is required to perform checkouts over HTTP from GitHub.
+
+      You may want to upgrade:
+        brew upgrade git
+
+    EOS
+  end
+end
+
 module Homebrew extend self
   def doctor
     old_stdout = $stdout
@@ -820,6 +842,7 @@ module Homebrew extend self
       check_missing_deps
       check_git_status
       check_for_leopard_ssl
+      check_git_version
     ensure
       $stdout = old_stdout
     end