Change the default value of command_requires_full_index from 1 to -1
(unset) and add an accessor that dies when the value has not been
explicitly set by the command.  This catches commands that read the
index without first declaring whether they need the full index
expanded.

Each command that has already been audited for sparse-index
compatibility sets command_requires_full_index = 0.  Commands that
have not been audited will now fail loudly instead of silently
expanding the index, making it clear which commands still need
attention.

Set GIT_ALLOW_SPARSE_INDEX_WITHOUT_DECLARATION=1 to fall back to the
old behavior (expand full index) while investigating failures.

These commands were found to have never declared command_requires_full_index.
After analysis, all four are safe with sparse index (= 0).

git-am: The patch application path uses index_name_pos() which auto-expands
sparse directory entries on demand when a specific file path is looked up.
The three-way merge fallback uses merge-ort, which works from tree OIDs and
handles sparse directories natively.  refresh_index() explicitly skips
S_ISSPARSEDIR entries.  git-rebase --apply invokes git-am as a child
process; rebase itself already declares command_requires_full_index = 0.
Tested by t1092.42 (merge, cherry-pick, and rebase) which exercises
rebase --apply through the sparse-index repo.

git-mv: For in-cone moves, all index lookups use index_name_pos() on
paths that cannot be ancestors of sparse directory entries (by
definition of the sparse cone), so no expansion is triggered.
index_range_of_same_dir() iterates only in-cone entries in this case.
For out-of-cone moves (--sparse flag), a targeted ensure_full_index()
is needed because the bulk cache[] iteration and rename_index_entry_at()
assume individual file entries, not collapsed directory stubs.
Add tests in t1092.59 (sparse-index is not expanded) to verify that
in-cone file and directory renames do not trigger index expansion.

git-submodule--helper: Submodule entries (S_ISGITLINK) are never collapsed
into sparse directory entries -- convert_to_sparse_rec() in sparse-index.c
explicitly blocks directories containing gitlinks from being collapsed.
module_list_compute() filters on S_ISGITLINK, skipping any sparse
directory entries.  module_add() already calls ensure_full_index()
unconditionally before its index iteration.
Tested by t1092.55 (submodule handling).

grep (submodule path): grep_cache() explicitly handles S_ISSPARSEDIR
entries by reading the tree object and recursing via grep_tree(), so
sparse directory contents are grepped correctly.  The subrepo index
is independent of the superproject.
Tested by t1092.85 (grep sparse directory within submodules).