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>

1 files changed, 17 insertions, 0 deletions

diff --git a/t/t7102-reset.sh b/t/t7102-reset.sh
index 601b2bf97f..d05426062e 100755
--- a/t/t7102-reset.sh
+++ b/t/t7102-reset.sh
@@ -472,6 +472,23 @@ test_expect_success '--mixed refreshes the index' '
 	test_cmp expect output
 '
 
+test_expect_success '--mixed preserves skip-worktree' '
+	echo 123 >>file2 &&
+	git add file2 &&
+	git update-index --skip-worktree file2 &&
+	git reset --mixed HEAD >output &&
+	test_must_be_empty output &&
+
+	cat >expect <<-\EOF &&
+	Unstaged changes after reset:
+	M	file2
+	EOF
+	git update-index --no-skip-worktree file2 &&
+	git add file2 &&
+	git reset --mixed HEAD >output &&
+	test_cmp expect output
+'
+
 test_expect_success 'resetting specific path that is unmerged' '
 	git rm --cached file2 &&
 	F1=$(git rev-parse HEAD:file1) &&