Don't glob when no directories are specified.

Before this commit when no directories were specified for the FileScanner class
(or no existing ones) the code would end up globbing for /**/*.rb. That is, it
would search for all Ruby source files starting in the root directory of your
disk. Yikes.

This commit fixes this by simply not globbing at all if the list of directories
is empty.

See #83 and #84 for more information.

lib/ruby-lint/file_scanner.rb

@@ -60,9 +60,7 @@ def initialize(directories = self.class.default_directories, ignore = [])
     # @return [Array]
     #
     def scan(constant)
-      # Globbing all files at once and then comparing those results is faster
-      # than running a Dir.glob for every call to #scan.
-      @glob_cache ||= Dir.glob("#{directories.join(',')}/**/*.rb")
+      @glob_cache ||= directories.empty? ? [] : glob_ruby_files
 
       unless constant_paths_cached?(constant)
         build_constant_paths_cache(constant)
@@ -73,6 +71,13 @@ def scan(constant)
 
     private
 
+    ##
+    # @return [Array]
+    #
+    def glob_ruby_files
+      return Dir.glob("#{directories.join(',')}/**/*.rb")
+    end
+
     ##
     # Searches all the files that could potentially define the given constant
     # and caches them.

spec/ruby-lint/file_scanner_spec.rb

@@ -61,4 +61,12 @@
 
     scanner.scan('Example::User').empty?.should == true
   end
+
+  example 'do not scan when there are no directories' do
+    scanner = RubyLint::FileScanner.new([])
+
+    scanner.should_not receive(:glob_ruby_files)
+
+    scanner.scan('Foo')
+  end
 end