convert svn access to github to git sparse checkout

[jedwards4b]

Reinterprets svn access to github.com as a git sparse checkout.

User interface changes?: Yes
I could not figure out how to maintain the original paths in the checkout so this will require changes in components who have used
svn access to github to do sparse checkout in the past.

Fixes: #203

Testing:
test removed:
unit tests: all pass
system tests: suprepo tests are failing - this is issue #202 test_container_simple_svn also fails (new)
manual testing: Checkout cam using cam6_3_130

[jedwards4b]

If we can figure out how to maintain the original paths in the checkout this PR could potentially allow us to maintain the current cam structure in the face of the upcoming change in support for svn access to github.com

[gold2718]

If we can figure out how to maintain the original paths in the checkout this PR could potentially allow us to maintain the current cam structure in the face of the upcoming change in support for svn access to github.com

I'm pretty sure any attempt to do this will break all the git internals. Even the index becomes useless so manage_externals could not even get a status on such a repo.