Update to new libgit version

Change-Id: Ice3ae02650d679d873cb76b2529b1d4e9bce0afb
diff --git a/vendor/libgit2 b/vendor/libgit2
index 15e1193..25a4b97 160000
--- a/vendor/libgit2
+++ b/vendor/libgit2
@@ -1 +1 @@
-Subproject commit 15e119375018fba121cf58e02a9f17fe22df0df8
+Subproject commit 25a4b97b1bd0b5316deb77866756a3db772e08a2