diff_file: test empty workdir file grows after obtaining a diff

This test was also provided by @jorio
https://github.com/libgit2/libgit2/pull/6208#issuecomment-1034072050

tests/diff/externalmodifications.c

@@ -96,7 +96,7 @@ void test_diff_externalmodifications__file_deleted(void)
 
 	/* Delete the file */
 	cl_git_rmfile(path.ptr);
-	
+
 	/* Attempt to get a patch */
 	cl_git_fail(git_patch_from_diff(&patch, diff, 0));
 
@@ -104,3 +104,30 @@ void test_diff_externalmodifications__file_deleted(void)
 	git_diff_free(diff);
 	git_str_dispose(&path);
 }
+
+void test_diff_externalmodifications__empty_file_becomes_non_empty(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_patch* patch;
+	git_str path = GIT_STR_INIT;
+
+	cl_git_pass(git_str_joinpath(&path, git_repository_workdir(g_repo), "README"));
+
+	/* Empty out the file */
+	cl_git_mkfile(path.ptr, "");
+
+	/* Get a diff */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(1, git_diff_num_deltas(diff));
+	cl_assert_equal_i(0, git_diff_get_delta(diff, 0)->new_file.size);
+
+	/* Simulate file modification after we've gotten the diff */
+	cl_git_mkfile(path.ptr, "hello");
+	cl_git_fail(git_patch_from_diff(&patch, diff, 0));
+
+	git_index_free(index);
+	git_diff_free(diff);
+	git_str_dispose(&path);
+}