(PUP-1846) File content diffing should respect loglevel

The current behavior is to log all diff output on file content changes
(if show_diff is set) at the info level, rather than the loglevel of the
resource.

This change alters the behavior to log at the same loglevel as the file
resource.
Merge branch 'pr/2410'

* pr/2410:
  (PUP-1846) File content diffing should respect loglevel