Update to latest libgit2

Specifically to changes made by ligbit2's pull-request number #1115:

https://github.com/libgit2/libgit2/pull/1115

src/pygit2/diff.c

@@ -360,7 +360,7 @@ PyObject *
 Diff_find_similar(Diff *self, PyObject *args)
 {
     int err;
-    git_diff_find_options opts = {0};
+    git_diff_find_options opts = GIT_DIFF_FIND_OPTIONS_INIT;
 
     if (!PyArg_ParseTuple(args, "|i", &opts.flags))
         return NULL;

src/pygit2/index.c

@@ -105,7 +105,7 @@ Index_clear(Index *self)
 PyObject *
 Index_diff_tree(Index *self, PyObject *args)
 {
-    git_diff_options opts = {0};
+    git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
     git_diff_list *diff;
     int err;
 

src/pygit2/tree.c

@@ -269,7 +269,7 @@ Tree_getitem(Tree *self, PyObject *value)
 PyObject *
 Tree_diff_tree(Tree *self, PyObject *args)
 {
-    git_diff_options opts = {0};
+    git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
     git_diff_list *diff;
     int err;