ARGV: add .dry_run? method

Add an ARGV.dry_run? helper method for '--dry-run'/'-n' now that
'-n' is being used as a git-style dry-run in two commands.

Closes Homebrew/homebrew#12898.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Library/Homebrew/cmd/cleanup.rb

@@ -16,7 +16,7 @@ module Homebrew extend self
       end
       clean_cache
       # seems like a good time to do some additional cleanup
-      Homebrew.prune unless ARGV.switch? 'n'
+      Homebrew.prune unless ARGV.dry_run?
     else
       ARGV.formulae.each do |f|
         cleanup_formula f
@@ -39,7 +39,7 @@ module Homebrew extend self
       f.rack.children.each do |keg|
         if f.installed_prefix != keg
           puts "Removing #{keg}..."
-          rm_rf keg unless ARGV.switch? 'n'
+          rm_rf keg unless ARGV.dry_run?
         end
       end
     elsif f.rack.children.length > 1
@@ -59,7 +59,7 @@ module Homebrew extend self
         old_bottle = bottle_file_outdated? f, pn
         if not f or (f.version != version or ARGV.switch? "s" and not f.installed?) or old_bottle
           puts "Removing #{pn}..."
-          rm pn unless ARGV.switch? 'n'
+          rm pn unless ARGV.dry_run?
         end
       end
     end

Library/Homebrew/cmd/link.rb

@@ -9,12 +9,9 @@ module Homebrew extend self
       abort "Cowardly refusing to `sudo brew link'"
     end
 
-    if ARGV.force?
-      mode = :force
-    elsif ARGV.include?("--dry-run") || ARGV.include?("-n")
-      mode = :dryrun
-    else
-      mode = nil
+    if ARGV.force? then mode = :force
+    elsif ARGV.dry_run? then mode = :dryrun
+    else mode = nil
     end
 
     ARGV.kegs.each do |keg|

Library/Homebrew/extend/ARGV.rb

@@ -87,6 +87,9 @@ module HomebrewArgvExtension
   def one?
     flag? '--1'
   end
+  def dry_run?
+    include?('--dry-run') || switch?('n')
+  end
 
   def build_head?
     include? '--HEAD'