Mercurial: install extensions

Closes Homebrew#15839.

Library/Formula/mercurial.rb

@@ -9,7 +9,7 @@ class Mercurial < Formula
 
   depends_on 'docutils' => :python if build.head? or build.include? 'doc'
 
-  option 'doc', "Build the documentation. Depends on 'docutils' module"
+  option 'doc', "Build the documentation"
 
   def install
     # Don't add compiler specific flags so we can build against
@@ -22,7 +22,7 @@ def install
     system "make doc" if build.head? or build.include? 'doc'
     system "make local"
 
-    libexec.install 'hg', 'mercurial'
+    libexec.install 'hg', 'mercurial', 'hgext'
 
     # Symlink the hg binary into bin
     bin.install_symlink libexec/'hg'
@@ -39,15 +39,25 @@ def install
   end
 
   def caveats
-    if build.head? then <<-EOS.undent
+    s = ''
+
+    s += <<-EOS.undent
+      Extensions have been installed to:
+        #{libexec}/hgext
+    EOS
+
+    if build.head? then s += <<-EOS.undent
+
       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
+        brew cleanup mercurial && brew link mercurial
       EOS
     end
+
+    return s
   end
 
   def test