Gitlab::Git::Hook should set the GL_REPOSITORY environment variable

When introducing the GL_REPOSITORY variable we overlooked the Gitlab::Git::Hook class.

This has been causing PostReceive jobs to fail on dev.gitlab.org since gitlab-shell!136 (merged) was merged.

The GL_REPOSITORY variable should be among those set on the hook process.

Edited Jun 29, 2017 by Jacob Vosmaer