update: automatically tap migrated formulae.

When a formula is removed from core and installed let's tap it so users
can get updates without caring which tap we want to put things in.

This should allow us to migrate a lot of things to taps.

Closes Homebrew/homebrew#23760.

Library/Homebrew/cmd/update.rb

@@ -1,5 +1,6 @@
 require 'cmd/tap'
 require 'cmd/untap'
+require 'tap_migrations'
 
 module Homebrew extend self
   def update
@@ -55,6 +56,16 @@ module Homebrew extend self
     Homebrew.unlink_tap_formula(report.removed_tapped_formula)
     Homebrew.link_tap_formula(report.new_tapped_formula)
 
+    # automatically tap any migrated formulae's new tap
+    report.select_formula(:D).each do |f|
+      next unless Formula.factory(f).installed?
+      tap_user, tap_repo = TAP_MIGRATIONS[f].split '/'
+      begin
+        install_tap tap_user, tap_repo
+      rescue AlreadyTappedError => e
+      end
+    end
+
     if report.empty?
       puts "Already up-to-date."
     else