Use filenames as a definitive answer (#2006)

* Separate find_by_extension and find_by_filename
find_by_extension now takes a path as argument and not only the file extension.
Currently only find_by_extension is used as a strategy.

* Add find_by_filename as first strategy

lib/linguist/strategy/filename.rb

@@ -1,9 +1,10 @@
 module Linguist
   module Strategy
-    # Detects language based on filename and/or extension
+    # Detects language based on filename
     class Filename
       def self.call(blob, _)
-        Language.find_by_filename(blob.name.to_s)
+        name = blob.name.to_s
+        Language.find_by_filename(name)
       end
     end
   end