Write .gitattributes in binary mode to prevent Rails from converting ASCII-8BIT to UTF-8

Stan Hurequested to merge
write-gitattributes-raw into master

This avoids Sidekiq errors in the PostReceive task due to .gitattributes files having ISO-8859 characters, such as:

Encoding::UndefinedConversionError: "\xC3" from ASCII-8BIT to UTF-8

Closes gitlab-org/gitlab-ce#20647

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