Mercurial: revert to standalone install

This resolves issues with the hg binary being in the wrong place and
the docutils module being required to install.

Note that users who need mercurial installed as a Python module will
need to install it some other way.

Closes Homebrew/homebrew#20620.

Formula/mercurial.rb

@@ -7,43 +7,56 @@ class Mercurial < Formula
 
   head 'http://selenic.com/repo/hg', :using => :hg
 
-  depends_on :python => 'docutils'
+  depends_on 'docutils' => :python if build.head? or build.include? 'doc'
+
+  option 'doc', "Build the documentation"
 
   def install
-    python do
-      # Inside this python do block, the PYTHONPATH (and more) is alreay set up
-      if python.from_osx? && !MacOS::CLT.installed?
-        # Help castrated system python on Xcode find the Python.h:
-        # Setting CFLAGS does not work :-(
-        inreplace 'setup.py', 'get_python_inc()', "'#{python.incdir}'"
-      end
+    # Don't add compiler specific flags so we can build against
+    # System-provided Python.
+    ENV.minimal_optimization
 
-      # Man pages come pre-built in source releases
-      system "make doc"
-      system "make", "PREFIX=#{prefix}", "install"
+    bash_completion.install 'contrib/bash_completion' => 'hg-completion.bash'
+
+    system "make doc" if build.head? or build.include? 'doc'
+    system "make local"
+
+    libexec.install 'hg', 'mercurial', 'hgext'
+
+    # Symlink the hg binary into bin
+    bin.install_symlink libexec/'hg'
+
+    # Remove the hard-coded python invocation from hg
+    inreplace bin/'hg', %r[^#!.*$], '#!/usr/bin/env python'
+
+    # Install some contribs
+    bin.install 'contrib/hgk'
 
     # Install man pages
     man1.install 'doc/hg.1'
     man5.install 'doc/hgignore.5', 'doc/hgrc.5'
   end
 
-    # install the completion scripts
-    bash_completion.install 'contrib/bash_completion' => 'hg-completion.bash'
-    zsh_completion.install 'contrib/zsh_completion' => '_hg'
-  end
-
   def caveats
     s = ''
+
+    s += <<-EOS.undent
+      Extensions have been installed to:
+        #{opt_prefix}/libexec/hgext
+    EOS
+
     if build.head? then s += <<-EOS.undent
-        To install the --HEAD version of mercurial, you have to:
-          1. `brew install mercurial`  # so brew can use this to fetch sources!
-          2. `brew unlink mercurial`
-          3. `brew install mercurial --HEAD`
-          4. `brew cleanup mercurial`  # to remove the older non-HEAD version
+
+      Mercurial is required to fetch its own repository, so there are now two
+      installations of mercurial on this machine. If the previous installation
+      was done via Homebrew, the old version may need to be cleaned up and new
+      version linked:
+
+        brew cleanup mercurial && brew link mercurial
       EOS
     end
-    s += python.standard_caveats if python
-    s
+
+    return s
   end
 
   def test