Makefile: Allow standalone installation of bundled Git

It is not possible to install Gitaly's bundled Git executables without also installing Gitaly's own binaries. This is a problem for Omnibus: for the sake of improved caching it will build Git executables before building Gitaly such that they don't have to be rebuilt whenever there is a new Gitaly commit. This is done by just calling "make git", which installs the complete Git distribution without Gitaly-specific parts. With the upcoming migration from a Git distribution to bundled Git we'll also want to install the bundled Git version in this step, which is not easily done right now.

Add two new build targets build-bundled-git and install-bundled-git to allow this usecase.