File content diff with show_diff respects loglevel

Currently Puppet just spits File content diffs at the notice loglevel
regardless of what we've told it. That's a bit silly, considering we've
already told Puppet we may want another loglevel for a particular
resource.

This fixes that behavior.
Merge pull request #2 from github/show-diff-should-respect-loglevel

File content diff with show_diff respects loglevel