Don't install `git-gui`

An Omnibus build slave should not need the `git-gui` and some platforms
don't have tcl/tk installed and configured by default.
Ensure gettext's `msgfmt` is available for git install

Git needs this utility during configure and compilation.
Add test coverage to ensure FreeBSD's use's `gmake`
Simplify our generated `chruby-exec` script

* source `chruby.sh` if needed.
* ensure sub-shells are executed with `bash`.
Ensure `~/.bashrc` has been sourced on Ruby integration tests.

By default EL systems will not source `~/.bashrc` when using `su`.
Merge pull request #28 from opscode-cookbooks/schisamo/git-no-tcltk

Don't install git-gui
Write a sane $HOME/.gitconfig
Add spec for custom chruby-exec
Stub the build_user_home for consistency

Without this stub, the build_user_home will fail on OSX vs linux because
it is calculated based on the currently running user.