Update to latest libgit2 API renaming

See libgit2 commit 25743bd7c5f14f2287d9c4, where git_index_add_from_workdir
was renamed to git_index_add_bypath

src/pygit2/index.c

@@ -88,7 +88,7 @@ Index_add(Index *self, PyObject *args)
     if (!PyArg_ParseTuple(args, "s", &path))
         return NULL;
 
-    err = git_index_add_from_workdir(self->index, path);
+    err = git_index_add_bypath(self->index, path);
     if (err < 0)
         return Error_set_str(err, path);