reset: make sparse-aware (except --mixed)

reset: make sparse-aware (except --mixed)

Remove `ensure_full_index` guard on `prime_cache_tree` and update
`prime_cache_tree_rec` to correctly reconstruct sparse directory entries in
the cache tree. While processing a tree's entries, `prime_cache_tree_rec`
must determine whether a directory entry is sparse or not by searching for
it in the index (*without* expanding the index). If a matching sparse
directory index entry is found, no subtrees are added to the cache tree
entry and the entry count is set to 1 (representing the sparse directory
itself). Otherwise, the tree is assumed to not be sparse and its subtrees
are recursively added to the cache tree.

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