Don't set the GIT_* environment variables when the source is not a git repo
Don't set the GIT_* environment variables when the source is not a git repo

This is better than setting them to the empty string, because then environ.get
will prefer the default argument, which is how our currently recommended
jinja2 templated meta.yaml source looks like.

See #322.

shastings517/conda-build,sandhujasmine/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,shastings517/conda-build,frol/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,mwcraig/conda-build,ilastik/conda-build,shastings517/conda-build,rmcgibbo/conda-build,dan-blanchard/conda-build,dan-blanchard/conda-build,frol/conda-build,ilastik/conda-build,mwcraig/conda-build,frol/conda-build,ilastik/conda-build,dan-blanchard/conda-build,mwcraig/conda-build