Reduce nesting in 'brew up'.

bin/brew

@@ -163,7 +163,8 @@ begin
       end
 
   when 'up', 'update'
-    if system "/usr/bin/which -s git"
+    abort "Please `brew install git' first." unless system "/usr/bin/which -s git"
+
     require 'update'
     updater = RefreshBrew.new
     old_revision = updater.current_revision
@@ -189,9 +190,6 @@ begin
         puts "No formulae were updated." unless updater.pending_formulae_changes?
       end
     end
-    else
-      abort "Please `brew install git' first."
-    end
 
     when 'ln', 'link'
       ARGV.kegs.each {|keg| puts "#{keg.link} links created for #{keg}"}