bump libgit2 to latest from master

brianmario · brianmario · commit 987bacd9df47 · 2018-01-17T13:55:58.000-08:00
diff --git a/vendor/libgit2 b/vendor/libgit2
@@ -1 +1 @@
-Subproject commit d43974fb5cbfda6ec28fc92b83d03db0af73748b
+Subproject commit 4893a9c01c8da084eb995178f80d0d453109056e
