(#5171) Made filebucket able to perform diffs

It is now possible to ask the filebucket to diff two files using a URL
of the form:

https://puppet/production/file_bucket_file/md5/{first file hash}?diff_with={second file hash}

The returned diff is a string, the output of the "diff" command.

Paired-with: Paul Berry <paul@puppetlabs.com>