use Tap#install

Library/Homebrew/cmd/install.rb

@@ -16,7 +16,10 @@ module Homebrew
     ARGV.named.each do |name|
       if !File.exist?(name) && (name !~ HOMEBREW_CORE_FORMULA_REGEX) \
               && (name =~ HOMEBREW_TAP_FORMULA_REGEX || name =~ HOMEBREW_CASK_TAP_FORMULA_REGEX)
-        install_tap $1, $2
+        user = $1
+        repo = $2.sub(/^homebrew-/, "")
+        tap = Tap.fetch(user, repo)
+        tap.install
       end
     end unless ARGV.force?
 

Library/Homebrew/cmd/update.rb

@@ -82,12 +82,12 @@ module Homebrew
       next unless (dir = HOMEBREW_CELLAR/f).exist?
       migration = TAP_MIGRATIONS[f]
       next unless migration
-      tap_user, tap_repo = migration.split "/"
-      install_tap tap_user, tap_repo
+      tap = Tap.fetch(*migration.split("/"))
+      tap.install
       # update tap for each Tab
       tabs = dir.subdirs.map { |d| Tab.for_keg(Keg.new(d)) }
       next if tabs.first.source["tap"] != "Homebrew/homebrew"
-      tabs.each { |tab| tab.source["tap"] = "#{tap_user}/homebrew-#{tap_repo}" }
+      tabs.each { |tab| tab.source["tap"] = "#{tap.user}/homebrew-#{tap.repo}" }
       tabs.each(&:write)
     end if load_tap_migrations
 

Library/Homebrew/formula_installer.rb

@@ -8,7 +8,6 @@ require "caveats"
 require "cleaner"
 require "formula_cellar_checks"
 require "install_renamed"
-require "cmd/tap"
 require "cmd/postinstall"
 require "hooks/bottles"
 require "debrew"
@@ -115,10 +114,11 @@ class FormulaInstaller
     begin
       formula.recursive_dependencies.map(&:to_formula)
     rescue TapFormulaUnavailableError => e
-      if Homebrew.install_tap(e.user, e.repo)
-        retry
-      else
+      if e.tap.installed?
         raise
+      else
+        e.tap.install
+        retry
       end
     end
   rescue FormulaUnavailableError => e