Fixes #39046

Fixes gitaly#629

So that we don't have to worry about cleaning empty
directories later. This was brought up at:

https://gitlab.com/gitlab-org/gitlab-ce/issues/38689#note_42242988

This reverts commit 1d3c33b5.

When calling pre-receive, post-receive, and update hooks, add the GitLab
username as the GL_USERNAME environment variable.

This patch only handles cases where pushes are over http, or via
the web interface.  Later, we will address the ssh case.

Closes gitaly#601

Closes gitaly#562

If the ref doesn't exist, and the source branch is deleted, we can't get it back
easily. Previously, we ignored this error by shelling out, so replicate that
behaviour.

In gitlab-org/gitlab-ee!2976, we saw that a given OID could point
to a commit, which would cause the delta size check to fail.
Gitaly already returns nil if the OID isn't a blob, so this change
makes the Rugged implementation consistent.

Closes gitaly#524

This allows the current Gitaly migration to depend on less code outside
of the Gitlab::Git module