Zip Archiving for commits and comparisons

Wilken Haaserequested to merge
w.haase/gitlab_git:master into master

This Pullrequest contains 2 functions for zip-archiving the whole set of files changed in a comparison and in a commit. I will open another Merge Request for Gitlab itself and link it here which contains the corresponding gui funtions.

This Request is linked with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/226

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