stash: apply stash using 'merge_ort_nonrecursive()'

stash: apply stash using 'merge_ort_nonrecursive()'

Update 'stash' to use 'merge_ort_nonrecursive()' to apply a stash to the
current working tree. When 'git stash apply' was converted from its shell
script implementation to a builtin in 8a0fc8d19d (stash: convert apply to
builtin, 2019-02-25), 'merge_recursive_generic()' was used to merge a stash
into the working tree as part of 'git stash (apply|pop)'. However, with the
single merge base used in 'do_apply_stash()', the commit wrapping done by
'merge_recursive_generic()' is not only unnecessary, but misleading (the
*real* merge base is labeled "constructed merge base"). Therefore, a
non-recursive merge of the working tree, stashed tree, and stash base tree
is more appropriate.

There are two options for a non-recursive merge-then-update-worktree
function: 'merge_trees()' and 'merge_ort_nonrecursive()'. Use
'merge_ort_nonrecursive()' to align with the default merge strategy used by
'git merge' (6a5fb96672 (Change default merge backend from recursive to ort,
2021-08-04)) and, because merge-ort does not operate in-place on the index,
avoid unnecessary index expansion. Update tests in 't1092' verifying index
expansion for 'git stash' accordingly.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>