If multiple kegs, default to formula keg

If it exists, ARGV.kegs will return the Formula.prefix keg for each rack examined.

So for ARGV=[wget, foo] and the following Cellar (wget/1.11, wget/1.12, foo/1.0) you'll get [wget/1.12, foo/1.0] from ARGV.kegs provided 1.12 is the formula version of wget.

Library/Homebrew/extend/ARGV.rb

@@ -31,8 +31,16 @@ module HomebrewArgvExtension
       linked_keg_ref = HOMEBREW_REPOSITORY/"Library/LinkedKegs"/name
 
       if not linked_keg_ref.symlink?
-        raise MultipleVersionsInstalledError.new(name) if dirs.length > 1
+        if dirs.length == 1
-        Keg.new(dirs.first)
+          Keg.new(dirs.first)
+        else
+          prefix = Formula.factory(canonical_name).prefix
+          if prefix.directory?
+            Keg.new(prefix)
+          else
+            raise MultipleVersionsInstalledError.new(name)
+          end
+        end
       else
         Keg.new(linked_keg_ref.realpath)
       end