Provide a way to build with embedded libgit2

Currently gitaly project builds against the system libgit2 library. But unfortunately gitaly requires a specific version of the library. At this point, the libgit2 requirement is version 1.3.0, while the upstream stable version is 1.4.3.

And this creates a problem for Linux (and other OS) packagers who need to pin libgit2 system package to 1.3.0 and never update it. Which itself creates a problem for other projects that need newer libgit2.

Please consider building a specific libgit2 version during the build and then embed it into the gitaly binary. This way gitaly can be distributed as a single binary without dependency on a specific libgit2 system library.

This can be a build-time option to give users the possibility to choose one of the two options (system vs embeded libgit2).