remove migrate_taps

We don't need to migrate the tapped formulae from symlink-based to
directory-based structure any more.

Instead, we add core tap install check for `brew update-report` which
will be invoked by `brew update`.

Library/Homebrew/cmd/prune.rb

@@ -37,8 +37,6 @@ module Homebrew
       end
     end
 
-    migrate_taps :force => true unless ARGV.dry_run?
-
     if ObserverPathnameExtension.total.zero?
       puts "Nothing pruned" if ARGV.verbose?
     else

Library/Homebrew/cmd/tap.rb

@@ -4,7 +4,6 @@ module Homebrew
   def tap
     if ARGV.include? "--repair"
       Tap.each(&:link_manpages)
-      migrate_taps :force => true
     elsif ARGV.include? "--list-official"
       require "official_taps"
       puts OFFICIAL_TAPS.map { |t| "homebrew/#{t}" }
@@ -36,12 +35,4 @@ module Homebrew
       true
     end
   end
-
-  # Migrate tapped formulae from symlink-based to directory-based structure.
-  def migrate_taps(options = {})
-    ignore = HOMEBREW_LIBRARY/"Formula/.gitignore"
-    return unless ignore.exist? || options.fetch(:force, false)
-    (HOMEBREW_LIBRARY/"Formula").children.each { |c| c.unlink if c.symlink? }
-    ignore.unlink if ignore.exist?
-  end
 end

Library/Homebrew/cmd/update-report.rb

-require "cmd/tap"
 require "formula_versions"
 require "migrator"
 require "formulary"
@@ -6,8 +5,7 @@ require "descriptions"
 
 module Homebrew
   def update_report
-    # migrate to new directories based tap structure
-    migrate_taps
+    install_core_tap_if_necessary
 
     hub = ReporterHub.new
 
@@ -63,6 +61,15 @@ module Homebrew
   def shorten_revision(revision)
     Utils.popen_read("git", "-C", HOMEBREW_REPOSITORY, "rev-parse", "--short", revision).chomp
   end
+
+  def install_core_tap_if_necessary
+    core_tap = CoreTap.instance
+    return if core_tap.installed?
+    CoreTap.ensure_installed! :quiet => false
+    revision = core_tap.git_head
+    ENV["HOMEBREW_UPDATE_BEFORE_HOMEBREW_HOMEBREW_CORE"] = revision
+    ENV["HOMEBREW_UPDATE_AFTER_HOMEBREW_HOMEBREW_CORE"] = revision
+  end
 end
 
 class Reporter