(PUP-2073) fix for multiple diff_args

Previously, if you specified multiple arguments in the `diff_args`
setting, they would be considered a single argument and quoted to the
diff command. Puppet would then try to execute:

    diff '-u --strip-trailing-cr' pathA pathB

But would silently fail without showing any diffs.

This commit ensures we can handle multiple diff args.
Merge branch 'pr/2477'

* pr/2477:
  (PUP-2073) fix for multiple diff_args

Closes GH-2477