update-index: integrate with sparse index

update-index: integrate with sparse index

Enable use of the sparse index with `update-index`. Most variations of
`update-index` work without explicitly expanding the index or making any
other updates in or outside of `update-index.c`.

The one usage requiring additional changes is `--cacheinfo`; if a file
inside a sparse directory was specified, the index would not be expanded
until after the cache tree is invalidated, leading to a mismatch between the
index and cache tree. This scenario is handled by rearranging
`add_index_entry_with_check`, allowing `index_name_stage_pos` to expand the
index *before* attempting to invalidate the relevant cache tree path,
avoiding cache tree/index corruption.

Signed-off-by: Victoria Dye <vdye@github.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>