Replace remaining shelled-out whiches

Library/Homebrew/extend/ENV.rb

@@ -479,10 +479,10 @@ class << ENV
         EOS
       end
 
-    elsif `/usr/bin/which gfortran`.chuzzle
+    elsif which 'gfortran'
       ohai "Using Homebrew-provided fortran compiler."
       puts "This may be changed by setting the FC environment variable."
-      self['FC'] = `/usr/bin/which gfortran`.chomp
+      self['FC'] = which 'gfortran'
       self['F77'] = self['FC']
 
       fc_flag_vars.each {|key| self[key] = cflags}

Library/Homebrew/global.rb

@@ -46,7 +46,7 @@ undef cache # we use a function to prevent adding home_cache to the global scope
 HOMEBREW_CACHE_FORMULA = HOMEBREW_CACHE+"Formula"
 
 if not defined? HOMEBREW_BREW_FILE
-  HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] || `which brew`.chomp
+  HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] || which('brew').to_s
 end
 
 HOMEBREW_PREFIX = Pathname.new(HOMEBREW_BREW_FILE).dirname.parent # Where we link under

Library/brew.rb

@@ -91,7 +91,7 @@ begin
       ENV["HOMEBREW_#{e}"] = Object.const_get "HOMEBREW_#{e}"
     end
     exec "brew-#{cmd}", *ARGV
-  elsif require? `/usr/bin/which brew-#{cmd}.rb`
+  elsif require? which("brew-#{cmd}.rb").to_s
     exit 0
   else
     onoe "Unknown command: #{cmd}"