recipetool: create: fix handling of github URLs

recipetool: create: fix handling of github URLs

For a while now, Github hasn't been advertising a specific repository
URL since cloning the web URL with git works. Armed with this knowledge
and fully expecting people to just paste the github URL, we need to
handle this situation specially. If it looks like a github URL to the
root of a repository then treat it as a git repository instead of a
normal https URL to be fetched by the wget fetcher.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>