sparse-index: introduce GIT_TEST_FAIL_ENSURE_FULL_INDEX

[derrickstolee]

To be extra sure that our integrated commands do not expand in normal
cases, create a GIT_TEST_FAIL_ENSURE_FULL_INDEX that fails inside
ensure_full_index() if the repo settings indicate we are in an
integrated command. Enable this for the Scalar functional tests.

Inspired by a conversation with @dscho.

[derrickstolee]

This is functioning as expected, and gives me some directions to investigate. It is possible/probable that we are testing scenarios in the Scalar functional tests that require expanding the index. If not, then it gives directions for future improvement. (The first failure I saw involved git cherry-pick so we will see!)

[dscho]

I am really glad that my suggestion was worth the time to implement it!

[derrickstolee]

After digging into it, it appears that this is failing during a git sparse-checkout init --cone command, and it's during the index write due to the was_full protection (if the index was full on input, we expand it back out to a full one after the write). This means we are not ready to do such a global check. Even after #421 is merged we would still need to remove that was_full protection or inject a way to disable the environment variable at some points in the Scalar functional tests. That's not worth it right now.