fsmonitor: integrate with sparse index

fsmonitor: integrate with sparse index

If we need to expand a sparse-index into a full one, then the FS Monitor
bitmap is going to be incorrect. Ensure that we start fresh at such an
event.

While this is currently a performance drawback, the eventual hope of the
sparse-index feature is that these expansions will be rare and hence we
will be able to keep the FS Monitor data accurate across multiple Git
commands.

These tests are added to demonstrate that the behavior is the same
across a full index and a sparse index, but also that file modifications
to a tracked directory outside of the sparse cone will trigger
ensure_full_index().

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>