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