avoid load_blob and shebang check on files with extensions

previously, any file with an unrecognized file extension was loaded to
check for a shebang. now, this only occurs if the file has a generic
name with no file extension (like ./script)

it is possible this will no longer match certain scripts with esoteric
extensions (if we find these we can add them to the shebang_extname?
method). however, most common script extensions (.sh, .rb, .pl, etc)
will continue to work since the file extension takes precedence over the
shebang line.

lib/linguist/blob_helper.rb

@@ -142,6 +142,13 @@ module Linguist
       ['.module', '.install', '.test', '.inc'].include?(extname)
     end
 
+    # Public: Is the blob likely to have a shebang?
+    #
+    # Return true or false
+    def shebang_extname?
+      extname.empty?
+    end
+
     MEGABYTE = 1024 * 1024
 
     # Public: Is the blob too big to load?
@@ -592,6 +599,9 @@ module Linguist
     #
     # Returns the Language or nil
     def shebang_language
+      # Skip file extensions unlikely to have shebangs
+      return unless shebang_extname?
+
       if script = shebang_script
         Language[script]
       end

test/test_blob.rb

@@ -17,6 +17,12 @@ class TestBlob < Test::Unit::TestCase
     FileBlob.new(File.join(fixtures_path, name), fixtures_path)
   end
 
+  def script_blob(name)
+    blob = blob(name)
+    blob.instance_variable_set(:@name, 'script')
+    blob
+  end
+
   def test_name
     assert_equal "foo.rb", blob("foo.rb").name
   end
@@ -385,40 +391,40 @@ class TestBlob < Test::Unit::TestCase
   end
 
   def test_shebang_script
-    assert_equal 'sh', blob("script.sh").shebang_script
-    assert_equal 'bash', blob("script.bash").shebang_script
-    assert_equal 'zsh', blob("script.zsh").shebang_script
-    assert_equal 'perl', blob("script.pl").shebang_script
-    assert_equal 'ruby', blob("script.rb").shebang_script
-    assert_equal 'ruby', blob("script2.rb").shebang_script
-    assert_equal 'python', blob("script.py").shebang_script
-    assert_equal 'node', blob("script.js").shebang_script
-    assert_equal 'groovy', blob("script.groovy").shebang_script
-    assert_equal 'macruby', blob("script.mrb").shebang_script
-    assert_equal 'rake', blob("script.rake").shebang_script
-    assert_equal 'foo', blob("script.foo").shebang_script
-    assert_equal 'nush', blob("script.nu").shebang_script
-    assert_equal 'scala', blob("script.scala").shebang_script
-    assert_equal 'racket', blob("script.rkt").shebang_script
-    assert_equal nil, blob("foo.rb").shebang_script
+    assert_equal 'sh', script_blob("script.sh").shebang_script
+    assert_equal 'bash', script_blob("script.bash").shebang_script
+    assert_equal 'zsh', script_blob("script.zsh").shebang_script
+    assert_equal 'perl', script_blob("script.pl").shebang_script
+    assert_equal 'ruby', script_blob("script.rb").shebang_script
+    assert_equal 'ruby', script_blob("script2.rb").shebang_script
+    assert_equal 'python', script_blob("script.py").shebang_script
+    assert_equal 'node', script_blob("script.js").shebang_script
+    assert_equal 'groovy', script_blob("script.groovy").shebang_script
+    assert_equal 'macruby', script_blob("script.mrb").shebang_script
+    assert_equal 'rake', script_blob("script.rake").shebang_script
+    assert_equal 'foo', script_blob("script.foo").shebang_script
+    assert_equal 'nush', script_blob("script.nu").shebang_script
+    assert_equal 'scala', script_blob("script.scala").shebang_script
+    assert_equal 'racket', script_blob("script.rkt").shebang_script
+    assert_equal nil, script_blob("foo.rb").shebang_script
   end
 
   def test_shebang_language
-    assert_equal Language['Shell'], blob("script.sh").shebang_language
-    assert_equal Language['Shell'], blob("script.bash").shebang_language
-    assert_equal Language['Shell'], blob("script.zsh").shebang_language
-    assert_equal Language['Perl'], blob("script.pl").shebang_language
-    assert_equal Language['Ruby'], blob("script.rb").shebang_language
-    assert_equal Language['Python'], blob("script.py").shebang_language
-    assert_equal Language['JavaScript'], blob("script.js").shebang_language
-    assert_equal Language['Groovy'], blob("script.groovy").shebang_language
-    assert_equal Language['Ruby'], blob("script.mrb").shebang_language
-    assert_equal Language['Ruby'], blob("script.rake").shebang_language
-    assert_equal Language['Nu'], blob("script.nu").shebang_language
-    assert_equal Language['Scala'], blob("script.scala").shebang_language
-    assert_equal Language['Racket'], blob("script.rkt").shebang_language
-    assert_equal nil, blob("script.foo").shebang_language
-    assert_equal nil, blob("foo.rb").shebang_language
+    assert_equal Language['Shell'], script_blob("script.sh").shebang_language
+    assert_equal Language['Shell'], script_blob("script.bash").shebang_language
+    assert_equal Language['Shell'], script_blob("script.zsh").shebang_language
+    assert_equal Language['Perl'], script_blob("script.pl").shebang_language
+    assert_equal Language['Ruby'], script_blob("script.rb").shebang_language
+    assert_equal Language['Python'], script_blob("script.py").shebang_language
+    assert_equal Language['JavaScript'], script_blob("script.js").shebang_language
+    assert_equal Language['Groovy'], script_blob("script.groovy").shebang_language
+    assert_equal Language['Ruby'], script_blob("script.mrb").shebang_language
+    assert_equal Language['Ruby'], script_blob("script.rake").shebang_language
+    assert_equal Language['Nu'], script_blob("script.nu").shebang_language
+    assert_equal Language['Scala'], script_blob("script.scala").shebang_language
+    assert_equal Language['Racket'], script_blob("script.rkt").shebang_language
+    assert_equal nil, script_blob("script.foo").shebang_language
+    assert_equal nil, script_blob("foo.rb").shebang_language
   end
 
   def test_colorize