Removed cloning a specific revision from the mercurial download strategy.

When cloning a mercurial repository from a tagged revision, that tag definition
isn't actually included. This causes `hg archive -r tag_name` to fail.

Instead, just clone the repository's head and the `hg archive -r revision` will
handle getting the correct revision to for the build

Library/Homebrew/download_strategy.rb

@@ -243,9 +243,6 @@ class MercurialDownloadStrategy <AbstractDownloadStrategy
 
     unless @clone.exist?
       checkout_args = []
-      if (@spec == :revision) and @ref
-        checkout_args << '-r' << @ref
-      end
       checkout_args << url << @clone
       safe_system 'hg', 'clone', *checkout_args
     else