update libgit2 submodule to recent main (#704)

Stephan Dilly · web-flow · commit 1b1499a78dc0 · 2021-05-07T09:29:56.000-05:00
diff --git a/libgit2-sys/libgit2 b/libgit2-sys/libgit2
@@ -1 +1 @@
-Subproject commit f4b473fa842a84147d8036a27bac5c026c44dad4
+Subproject commit 4bd172087c30e09e7720a7df11cace47ee002256
