sparse-checkout: 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.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>

Author: Kevin Willford

merge-recursive.c

@@ -1546,7 +1546,7 @@ static int handle_change_delete(struct merge_options *opt,
 		 * path.  We could call update_file_flags() with update_cache=0
 		 * and update_wd=0, but that's a no-op.
 		 */
-		if (change_branch != opt->branch1 || alt_path)
+		if (change_branch != opt->branch1 || alt_path || !file_exists(update_path))
 			ret = update_file(opt, 0, changed, update_path);
 	}
 	free(alt_path);

t/t7615-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