reset: preserve skip-worktree bit in mixed reset

Change `update_index_from_diff` to set `skip-worktree` when applicable for new index entries. When `git reset --mixed <tree-ish>` is run, entries in the index with differences between the pre-reset HEAD and reset <tree-ish> are identified and handled with `update_index_from_diff`. For each file, a new cache entry in inserted into the index, created from the <tree-ish> side of the reset (without changing the working tree). However, the newly-created entry must have `skip-worktree` explicitly set in either of the following scenarios: 1. the file is in the current index and has `skip-worktree` set 2. the file is not in the current index but is outside of a defined sparse checkout definition Not setting the `skip-worktree` bit leads to likely-undesirable results for a user. It causes `skip-worktree` settings to disappear on the "diff"-containing files (but *only* the diff-containing files), leading to those files now showing modifications in `git status`. For example, when running `git reset --mixed` in a sparse checkout, some file entries outside of sparse checkout could show up as deleted, despite the user never deleting anything (and not wanting them on-disk anyway). Additionally, add a test to `t7102` to ensure `skip-worktree` is preserved in a basic `git reset --mixed` scenario and update a failure-documenting test from 19a0acc (t1092: test interesting sparse-checkout scenarios, 2021-01-23) with new expected behavior. Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Victoria Dye <vdye@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>