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
+    # Don't add compiler specific flags so we can build against
-      # Inside this python do block, the PYTHONPATH (and more) is alreay set up
+    # System-provided Python.
-      if python.from_osx? && !MacOS::CLT.installed?
+    ENV.minimal_optimization
-        # Help castrated system python on Xcode find the Python.h:
-        # Setting CFLAGS does not work :-(
-        inreplace 'setup.py', 'get_python_inc()', "'#{python.incdir}'"
-      end
 
-      # Man pages come pre-built in source releases
-      system "make doc"
-      system "make", "PREFIX=#{prefix}", "install"
-
-      # 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'
+
+    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
 
   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!
+      Mercurial is required to fetch its own repository, so there are now two
-          2. `brew unlink mercurial`
+      installations of mercurial on this machine. If the previous installation
-          3. `brew install mercurial --HEAD`
+      was done via Homebrew, the old version may need to be cleaned up and new
-          4. `brew cleanup mercurial`  # to remove the older non-HEAD version
+      version linked:
+
+        brew cleanup mercurial && brew link mercurial
       EOS
     end
-    s += python.standard_caveats if python
+
-    s
+    return s
   end
 
   def test