Doctor check for origin

Added checks on doctor.rb for:

* Missing git origin
* git origin not pointing at mxcl/homebrew

Tests use `git config --get remote.origin.url`.
Also added printout of origin to --config.rb.

Fixes Homebrew#14399.
Closes Homebrew#17117.

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

Library/Homebrew/cmd/--config.rb

@@ -54,6 +54,13 @@ def head
     if head.empty? then "(none)" else head end
   end
 
+  def origin
+    origin = HOMEBREW_REPOSITORY.cd do
+      `git config --get remote.origin.url`.chomp
+    end
+    if origin.empty? then "(none)" else origin end
+  end
+
   def describe_path path
     return "N/A" if path.nil?
     realpath = path.realpath
@@ -124,6 +131,7 @@ def write_build_config f
 
   def dump_verbose_config
     puts "HOMEBREW_VERSION: #{HOMEBREW_VERSION}"
+    puts "ORIGIN: #{origin}"
     puts "HEAD: #{head}"
     puts "HOMEBREW_PREFIX: #{HOMEBREW_PREFIX}"
     puts "HOMEBREW_CELLAR: #{HOMEBREW_CELLAR}"

Library/Homebrew/cmd/doctor.rb

@@ -721,6 +721,44 @@ def check_git_newline_settings
   end
 end
 
+def check_for_git_origin
+  return unless which "git"
+
+  HOMEBREW_REPOSITORY.cd do
+    if `git config --get remote.origin.url`.chomp.empty? then <<-EOS.undent
+      Missing git origin remote.
+
+      Without a correctly configured origin, Homebrew won't update
+      properly. You can solve this by adding the Homebrew remote:
+        cd #{HOMEBREW_REPOSITORY}
+        git add remote origin https://github.com/mxcl/homebrew.git
+      EOS
+    end
+  end
+end
+
+def check_the_git_origin
+  return unless which "git"
+  return if check_for_git_origin
+
+  HOMEBREW_REPOSITORY.cd do
+    origin = `git config --get remote.origin.url`.chomp
+
+    unless origin =~ /mxcl\/homebrew\.git$/ then <<-EOS.undent
+      Suspicious git origin remote found.
+
+      With a non-standard origin, Homebrew won't pull updates from
+      the main repository. The current git origin is:
+        #{origin}
+
+      Unless you have compelling reasons, consider setting the
+      origin remote to point at the main repository, located at:
+        https://github.com/mxcl/homebrew.git
+      EOS
+    end
+  end
+end
+
 def check_for_autoconf
   return unless MacOS::Xcode.provides_autotools?