This adds a `-Wunused-result`-proof `GIT_UNUSED()`, just to demonstrate
that it works. With this, sortedcache.h is now completely
`GIT_WARN_UNUSED_RESULT`-annotated!

The `git_index_free()` merely decrement the reference counter from 2 to
1, and does not "free" the index.
Thus, the following `git_repository_index()` merely increase the counter
to 2, instead of read index from disk.
The written index is not read and parsed, which makes this test case
effectively becomes a no-op.

We should always verify error codes returned by function calls in our
test suite to not accidentally miss any weird results. Coverity reported
missing checks in several locations, which this commit fixes.

Users may want to turn off core.protectNTFS, perhaps to import (and then
repair) a broken tree.  Ensure that core.protectNTFS=false is honored.

Enable core.protectNTFS by default everywhere and in every codepath, not
just on checkout.

Test that when we enable core.protectNTFS that we cannot add
platform-specific invalid paths to the index.

The name of the `add_invalid_filename` function suggests that we
_want_ to add an invalid filename.  Rename the function to show that
we expect to _fail_ to add the invalid filename.

Ensure that the new protection around .git::$INDEX_ALLOCATION rules are
enabled for adding to the index when core.protectNTFS is set.

The name of the `write_invalid_filename` function suggests that we
_want_ to write an invalid filename.  Rename the function to show that
we expect to _fail_ to write the invalid filename.