Revert "Removew brew-which"

This reverts commit c0a32c81eb1b176448cd41866f2d22427c729522.

Will add a deprecation notice and remove at a later point in time.

Library/Contributions/cmd/brew-which.rb

+require 'extend/pathname'
+
+module Homebrew
+  def which_versions which_brews=nil
+    brew_links = Array.new
+    version_map = Hash.new
+
+    real_cellar = HOMEBREW_CELLAR.realpath
+
+    (HOMEBREW_PREFIX/'opt').subdirs.each do |path|
+      next unless path.symlink? && path.resolved_path_exists?
+      brew_links << Pathname.new(path.realpath)
+    end
+
+    brew_links = brew_links.collect{|p|p.relative_path_from(real_cellar).to_s}.reject{|p|p.start_with?("../")}
+
+    brew_links.each do |p|
+      parts = p.split("/")
+      next if parts.count < 2 # Shouldn't happen for normally installed brews
+      brew = parts.shift
+      version = parts.shift
+
+      next unless which_brews.include? brew if which_brews
+
+      versions = version_map[brew] || []
+      versions << version unless versions.include? version
+      version_map[brew] = versions
+    end
+
+    return version_map
+  end
+
+  def which
+    which_brews = ARGV.named.empty? ? nil : ARGV.named
+
+    brews = which_versions which_brews
+    brews.keys.sort.each do |b|
+      puts "#{b}: #{brews[b].sort*' '}"
+    end
+    puts
+  end
+end
+
+Homebrew.which