Added GL_PROTOCOL as an ENV variable to gitCommand

With this MR we are now populating an ENV variable called `GL_PROTOCOL` when calling the `gitCommand` method. This will allow GitLab to allow or deny the request based on the allowed Git access protocols defined in the Application Settings.

This MR is needed for:

gitlab-org/gitlab-shell!62 

gitlab-org/gitlab-ce!4696

gitlab-org/gitlab-ce#18601

See merge request !51

Add link to "brief history" blog post in README



See merge request !50

Workhorse to serve the output for format-patch

Now this is done by Rails which is sad for the Unicorn worker, so this
is moved here. This commit excludes tests for now, which will be added
after the weekend.

Part of fixing: gitlab-org/gitlab-ce#13999

See merge request !48

Also a forgotten diff is now renamed to patch

Use docker go image

By using the official go docker image there is no need to install go
manually and this saves time pulling the ruby image which is not being
used anyway. Also, some time is save now not all packages need to be
installed to install Go.

At least thats what I expected when I typed the commit message, the speed gain is to be neglected though at least this yields a clean `.gitlab-ci.yml`.

See merge request !49

By using the official go docker image there is no need to install go
manually and this saves time pulling the ruby image which is not being
used anyway. Also, some time is save now not all packages need to be
installed to install Go.

Now this is done by Rails which is sad for the Unicorn worker, so this
is moved here. This commit excludes tests for now, which will be added
after the weekend.

Diffs served by workhorse

First iteration; mainly pushed the branch to trigger a build.

Based on `archive.go` and `blob.go`. 

TODO:
- [x] Add the correct route
- [x] Test locally
- [x] Write tests

See merge request !45

Send timestamps when proxying

For https://gitlab.com/gitlab-com/operations/issues/264

Companion to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4278

See merge request !47

Use Go 1.6.2



See merge request !46

Purposed of this commit is to trigger CI and hope it doesn't get the
'importing internal packages not allowed' error Im getting while
building.

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/33

This reverts commit 1fa69c8d. It
turned out that the new behavior broke 'shallow git clone'.

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/36

Be compatible with GOPATH



See merge request !43

Delay HTTP headers for Git HTTP responses

Alternative to https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/38
and https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/40 .

See merge request !42