apply: Test for git_apply_to_tree failures when new files are added

Introduce an unit test to validate if git_apply_to_tree() fails when an
applied patch adds new files.

tests/apply/tree.c

 #include "clar_libgit2.h"
+#include "apply_helpers.h"
 #include "../merge/merge_helpers.h"
 
 static git_repository *repo;
@@ -56,3 +57,38 @@ void test_apply_tree__one(void)
 	git_commit_free(b_commit);
 }
 
+void test_apply_tree__adds_file(void)
+{
+	git_oid a_oid;
+	git_commit *a_commit;
+	git_tree *a_tree;
+	git_diff *diff;
+	git_index *index = NULL;
+
+	struct merge_index_entry expected[] = {
+		{ 0100644, "f51658077d85f2264fa179b4d0848268cb3475c3", 0, "asparagus.txt" },
+		{ 0100644, "68f6182f4c85d39e1309d97c7e456156dc9c0096", 0, "beef.txt" },
+		{ 0100644, "4b7c5650008b2e747fe1809eeb5a1dde0e80850a", 0, "bouilli.txt" },
+		{ 0100644, "c4e6cca3ec6ae0148ed231f97257df8c311e015f", 0, "gravy.txt" },
+		{ 0100644, "6370543fcfedb3e6516ec53b06158f3687dc1447", 0, "newfile.txt" },
+		{ 0100644, "68af1fc7407fd9addf1701a87eb1c95c7494c598", 0, "oyster.txt" },
+		{ 0100644, "94d2c01087f48213bd157222d54edfefd77c9bba", 0, "veal.txt" },
+	};
+
+	git_oid_fromstr(&a_oid, "539bd011c4822c560c1d17cab095006b7a10f707");
+
+	cl_git_pass(git_commit_lookup(&a_commit, repo, &a_oid));
+
+	cl_git_pass(git_commit_tree(&a_tree, a_commit));
+
+	cl_git_pass(git_diff_from_buffer(&diff,
+		DIFF_ADD_FILE, strlen(DIFF_ADD_FILE)));
+
+	cl_git_pass(git_apply_to_tree(&index, repo, a_tree, diff, NULL));
+	merge_test_index(index, expected, 7);
+
+	git_diff_free(diff);
+	git_tree_free(a_tree);
+	git_commit_free(a_commit);
+}
+