bump libgit2 to master

alanjrogers · alanjrogers · commit 9bd9d5c22115 · 2014-10-24T08:28:59.000+11:00
diff --git a/External/libgit2 b/External/libgit2
@@ -1 +1 @@
-Subproject commit a7b2cd4257d921b5b81991bf7982716f4711ab3d
+Subproject commit 4bb6ffb6bb8ad9d49eb173350be965183cd09c96
