Add git2go dependency

For some operations we cannot use git command directly because it will require exposing sensitive informations through command line. This can be viewed for example via ps.

This PR adds git2go (go binding for libgit) as a dependency, creates new binary that allows to add new remote to the repository but it accepts remote URI on standard input instead of command line. For now it's only compiled and tested in gitaly CI. This will allow to easier align with all installation methods.

Part of #1465 (closed)