bazlets: Replace native.git_repository with skylark rule

The native.git_repository rule does not work properly when invoked in a
`git submodule foreach` command. Bazel will deprecate the native rules
anyway, so replace with skylark's git_repository.

See [1] for context.

[1] bazelbuild/bazel#6358 (comment)

Change-Id: I97492855b99991e9812b07ed2851a31d67be3aea

bazlets.bzl

@@ -1,10 +1,12 @@
+load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
+
 NAME = "com_googlesource_gerrit_bazlets"
 
 def load_bazlets(
         commit,
         local_path = None):
     if not local_path:
-        native.git_repository(
+        git_repository(
             name = NAME,
             remote = "https://gerrit.googlesource.com/bazlets",
             commit = commit,