Use a temporary file for buffer for realtime diffs

`git diff` doesn't perform EOL conversion on stdin, causing it to
mistakenly flag every line as having changed when the working tree uses
a different EOL than the blobs. Writing the buffer to a temporary file
and diffing against that avoids this issue.

Fixes #232.

autoload/gitgutter/utility.vim

@@ -47,6 +47,10 @@ function! gitgutter#utility#filename()
   return fnamemodify(s:file, ':t')
 endfunction
 
+function! gitgutter#utility#extension()
+  return fnamemodify(s:file, ':e')
+endfunction
+
 function! gitgutter#utility#directory_of_file()
   return fnamemodify(s:file, ':h')
 endfunction