tap various commands: use tap name method.

Instead of hard coded `Homebrew/homebrew`

Library/Homebrew/cmd/tap-info.rb

@@ -10,7 +10,7 @@ module Homebrew
       end
     end
 
-    raise "Homebrew/homebrew is not allowed" if taps.any?(&:core_tap?)
+    raise "#{tap} is not allowed" if taps.any?(&:core_tap?)
 
     if ARGV.json == "v1"
       print_tap_json(taps)

Library/Homebrew/cmd/tap-pin.rb

@@ -4,9 +4,9 @@ module Homebrew
   def tap_pin
     ARGV.named.each do |name|
       tap = Tap.fetch(name)
-      raise "Homebrew/homebrew is not allowed" if tap.core_tap?
+      raise "#{tap} is not allowed" if tap.core_tap?
       tap.pin
-      ohai "Pinned #{tap.name}"
+      ohai "Pinned #{tap}"
     end
   end
 end

Library/Homebrew/cmd/tap-unpin.rb

@@ -4,9 +4,9 @@ module Homebrew
   def tap_unpin
     ARGV.named.each do |name|
       tap = Tap.fetch(name)
-      raise "Homebrew/homebrew is not allowed" if tap.core_tap?
+      raise "#{tap} is not allowed" if tap.core_tap?
       tap.unpin
-      ohai "Unpinned #{tap.name}"
+      ohai "Unpinned #{tap}"
     end
   end
 end

Library/Homebrew/cmd/untap.rb

@@ -6,7 +6,7 @@ module Homebrew
 
     ARGV.named.each do |tapname|
       tap = Tap.fetch(tapname)
-      raise "Homebrew/homebrew is not allowed" if tap.core_tap?
+      raise "#{tap} is not allowed" if tap.core_tap?
       tap.uninstall
     end
   end