Do not upload files with identical etags

[denisdefreyne]

This is continued off #480.

This is quite tricky to write tests for. Not sure how to handle that.

[denisdefreyne]

CC @nanoc/contributors — would like a +1

[mpapis]

yes looks good 👍, I guess my only remark is that the run grows quite big, but it's not related to this PR

[denisdefreyne]

Will look into adding some tests for this. (Will be hard, I guess!)

[mpapis]

avoid calculating local hash if remote is not available:

remote_etag = etags[key]
if remote_etag
  local_etag = calc_local_etag(file_path)
  next if remote_etag == local_etag
end

[denisdefreyne]

Thanks, updated! (see #552—not this PR)

[mpapis]

to simplify testing you could extract https://github.com/nanoc/nanoc/blob/aws-deploy-md5-v2/lib/nanoc/extra/deployers/fog.rb#L68-L86 to separate method upload - this also solves problem of growing run - and add attr_accessor :directory, :etags, :keys_to_destroy - this way you can check your custom directory and list of etags without actually connecting to aws.