Add explicit check for directory entry mode, throwing an error if found.
Without the check, sparse directory cache entries could be added to any
index (including non-sparse). In a sparse index, this would at least cause
inconsistencies in the cache tree; for a non-sparse index, such an entry
being present at all is a bug.

Signed-off-by: Victoria Dye <vdye@github.com>

In preparation of integrating 'git update-index' with the sparse index,
introduce tests for a variety of 'git update-index' uses. Some (namely those
in update-index add/remove`) are focused specifically on how 'git stash' uses
'git update-index' as a subcommand.

Signed-off-by: Victoria Dye <vdye@github.com>

Enable usage of the sparse index with `update-index`. Most variations of
`update-index` work without explicitly expanding the index (due to the
implicit expansion performed when attempting to find an existing entry).
However, in order to prevent manually creating a sparse directory cache
entry (current behavior is to throw an error when presented with a directory
using `--cacheinfo`), the index is expanded before the the entry is added.


Signed-off-by: Victoria Dye <vdye@github.com>

Rearrange `add_index_entry_with_check` to allow `index_name_stage_pos` to
expand the index _before_ attempting to invalidate the relevant cache tree
path. This permits implicit index expansion when adding a cache entry
outside the sparse checkout definition.

Signed-off-by: Victoria Dye <vdye@github.com>

Instead of ensuring the full index, only need to prevent reupdating sparse
directory entries to maintain expected behavior. Corresponding addition to
index expansion test verifies the sparse index is not expanded.

Signed-off-by: Victoria Dye <vdye@github.com>