update libgit2 submodule to recent main (rust-lang#708)

staktrace · web-flow · commit 0a64419db9fd · 2021-05-17T21:21:29.000-05:00
This is specifically to pick up the fix at libgit2/libgit2#5839
diff --git a/libgit2-sys/libgit2 b/libgit2-sys/libgit2
@@ -1 +1 @@
-Subproject commit 4bd172087c30e09e7720a7df11cace47ee002256
+Subproject commit be95f684da5d05e59919448c1061febbd4ff6eb8
