Respect changes to sparse_paths on update

[ksharma-bdai]

This patch addresses issue #265 by respecting updates to sparse_paths during gitman install, including:

• modifications to an existing sparse_paths list (additions or deletions)
• adding a new sparse_paths list to a previously full checkout
• removing all sparse_paths and reverting to a full checkout

This is done without using the git sparse-checkout command and should be compatible with git versions older than 2.25 where it was introduced.

This patch was also tested against a repository whose folder structure changed between revisions and the new sparse_paths was properly respected during update.

[ksharma-bdai]

Happy to make any style/structure changes - I tried to follow the conventions in the rest of git.py and the existing sparse-paths handling during install.

I ran make check and make test, all of the scenarios have tests and the new code is covered in code coverage - let me know if I missed anything else.

[ksharma-bdai]

Update: After a lot of trial and error it seems like the new tests for switching between sparse modes/etc are causing the tests to run into each other during execution, e.g. per this log with bad debug it looks a lot like both versions of it_merges_sources are running at the same time-ish, resulting in one failing since they both expect 3 checkouts but use slightly different repo names (one has submodules). I'll look into this more later.

[ksharma-bdai]

Abandoning this one to switch over to git sparse-checkout, will see if tests run into the same concurrency issues on the new PR.