git: update to v2.20.0

Update to git version v2.20.0. Required changes follow upstream commits:

* 00436bf1b1c2a8fe6cf5d2c2457d419d683042f4
  (archive: initialize archivers earlier)

* 611e42a5980a3a9f8bb3b1b49c1abde63c7a191e
  (xdiff: provide a separate emit callback for hunks)

Signed-off-by: Christian Hesse <mail@eworm.de>
diff --git a/ui-snapshot.c b/ui-snapshot.c
index 85efe64..9461d51 100644
--- a/ui-snapshot.c
+++ b/ui-snapshot.c
@@ -156,6 +156,7 @@
 	ctx.page.mimetype = xstrdup(format->mimetype);
 	ctx.page.filename = xstrdup(filename);
 	cgit_print_http_headers();
+	init_archivers();
 	format->write_func(hex, prefix);
 	return 0;
 }