Handle "git archive" via headers

Companion to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2675

See merge request !36

Unset Content-Length for raw Git blobs

Fixes https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/30#note_3560130

Failure to get raw Git blob via /api/v3 with error:

error: SendBlob: copy git cat-file stdout: Conn.Write wrote more than the declared Content-Length

See merge request !37

Blobs via workhorse

Works together with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2451

See merge request !30

This is meant as an extra layer of defense against untrusted user
input.

Fixed typo



See merge request !35

Add missing entries in build artifacts archive metadata

We need full directory structure, but since ZIP does not require it, we
need to calculate missing entries for directories when we generate
archive metadata.

Closes gitlab-org/gitlab-ce#12634

cc @jacobvosmaer @ayufan 

See merge request !34