Use shallow clones for submodules

This reduces the amount of git history downloaded from ~67M to ~11M.

Author: jyn514

src/bootstrap/bootstrap.py

@@ -990,7 +990,7 @@ def update_submodule(self, module, checked_out, recorded_submodules):
         run(["git", "submodule", "-q", "sync", module],
             cwd=self.rust_root, verbose=self.verbose)
 
-        update_args = ["git", "submodule", "update", "--init", "--recursive"]
+        update_args = ["git", "submodule", "update", "--init", "--recursive", "--depth=1"]
         if self.git_version >= distutils.version.LooseVersion("2.11.0"):
             update_args.append("--progress")
         update_args.append(module)