Make nacl recipes explicitly set GIT_MODE

scripts/slave/recipes/nacl.expected/linux_triggering.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.nacl",
       "--builder",
       "precise_64-newlib-arm_qemu-pnacl-dbg",
       "--slave",
       "TestSlave",
       "--spec",
-      "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'native_client', 'url': 'https://chromium.googlesource.com/native_client/src/native_client.git'}]",
+      "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'native_client', 'url': 'https://chromium.googlesource.com/native_client/src/native_client.git'}]",
       "--root",
       "native_client",
       "--revision_mapping_file",
       "{\"native_client\": \"got_revision\"}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "native_client@abcd",
@@ skipping 76 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@oneiric_32-newlib-arm_hw-pnacl-panda-dbg@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]