reset: add test demonstrating mixed reset outside sparse checkout

Mixed reset in a sparse checkout (cone mode or otherwise) will, if a file
outside of the sparse definition has differences between the working tree
and target, write the file to the local working copy of the repository.
Since this behavior conflicts with the typical expectations of a strict
checkout definition/cone, a test is added demonstrating (and explaining) it.

Signed-off-by: Victoria Dye <vdye@github.com>

Author: vdye

t/t1092-sparse-checkout-compatibility.sh

@@ -629,6 +629,32 @@ test_expect_success 'reset with wildcard pathspec' '
 	test_all_match git ls-files -s -- folder1
 '
 
+# NEEDSWORK: with mixed reset, files with differences between HEAD and <commit>
+# will be added to the work tree even if outside the sparse checkout
+# definition, and even if the file is modified to a state of having no local
+# changes. The file is "re-ignored" if a hard reset is executed. We may want to
+# change this behavior in the future and enforce that files are not written
+# outside of the sparse checkout definition.
+test_expect_success 'checkout and mixed reset file tracking [sparse]' '
+	init_repos &&
+
+	test_all_match git checkout -b reset-test update-deep &&
+	test_all_match git reset update-folder1 &&
+	test_all_match git reset update-deep &&
+
+	# At this point, there are no changes in the working tree. However,
+	# folder1/a now exists locally (even though it is outside of the sparse
+	# paths).
+	run_on_sparse test_path_exists folder1 &&
+
+	run_on_all rm folder1/a &&
+	test_all_match git status --porcelain=v2 &&
+
+	test_all_match git reset --hard update-deep &&
+	run_on_sparse test_path_is_missing folder1 &&
+	test_path_exists full-checkout/folder1
+'
+
 test_expect_success 'merge, cherry-pick, and rebase' '
 	init_repos &&