build: Have `bootstrap' run all the necessary steps.

* bootstrap: Always run "git submodule init".  Run
  `nix/sync-with-upstream'.
  Reported by Nikita Karetnikov and Andreas Enge.

bootstrap

@@ -7,10 +7,9 @@ set -e -x
 top_srcdir="$PWD"
 export top_srcdir
 
-if [ ! -d nix-upstream ]
-then
-    git submodule init
-fi
+git submodule init
 git submodule update
 
+./nix/sync-with-upstream
+
 exec autoreconf -vfi