sparse-checkout: make sure to update files with a modify/delete conflict

When using the sparse-checkout feature, the file might not be on disk
because the skip-worktree bit is on. This used to be a bug in the
(hence deleted) `recursive` strategy. Let's ensure that this bug does
not resurface.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: Kevin Willford

t/meson.build

@@ -940,6 +940,7 @@ integration_tests = [
   't7612-merge-verify-signatures.sh',
   't7614-merge-signoff.sh',
   't7615-diff-algo-with-mergy-operations.sh',
+  't7616-merge-sparse-checkout.sh',
   't7700-repack.sh',
   't7701-repack-unpack-unreachable.sh',
   't7702-repack-cyclic-alternate.sh',

t/t7616-merge-sparse-checkout.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+test_description='merge can handle sparse-checkout'
+
+. ./test-lib.sh
+
+# merges with conflicts
+
+test_expect_success 'setup' '
+	git branch -M main &&
+	test_commit a &&
+	test_commit file &&
+	git checkout -b delete-file &&
+	git rm file.t &&
+	test_tick &&
+	git commit -m "remove file" &&
+	git checkout main &&
+	test_commit modify file.t changed
+'
+
+test_expect_success 'merge conflict deleted file and modified' '
+	echo "/a.t" >.git/info/sparse-checkout &&
+	test_config core.sparsecheckout true &&
+	git checkout -f &&
+	test_path_is_missing file.t &&
+	test_must_fail git merge delete-file &&
+	test_path_is_file file.t &&
+	test "changed" = "$(cat file.t)"
+'
+
+test_done