Downstream impact from https://codereview.chromium.org/1693993002/

scripts/slave/recipes/libvpx/unittests.expected/basic_linux_64.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys, shutil\nroot = sys.argv[1]\nnuke_dirs = sys.argv[2:]\nfor fname in os.listdir(root):\n  path = os.path.join(root, fname)\n  if os.path.isfile(path):\n    os.unlink(path)\n  elif fname in nuke_dirs:\n    shutil.rmtree(path)\n",
       "[SLAVE_BUILD]",
       "libs",
       "obj",
       "vp8",
@@ skipping 51 matching lines @@
       "checkout",
       "-f",
       "FETCH_HEAD"
     ],
     "cwd": "[SLAVE_BUILD]/libvpx",
     "name": "git checkout"
   },
   {
     "cmd": [
       "git",
+      "rev-parse",
+      "HEAD"
+    ],
+    "cwd": "[SLAVE_BUILD]/libvpx",
+    "name": "read revision",
+    "stdout": "/path/to/tmp/",
+    "~followup_annotations": [
+      "@@@STEP_TEXT@<br/>checked out 'deadbeef'<br/>@@@"
+    ]
+  },
+  {
+    "cmd": [
+      "git",
       "clean",
       "-f",
       "-d",
       "-x"
     ],
     "cwd": "[SLAVE_BUILD]/libvpx",
     "name": "git clean"
   },
   {
     "cmd": [
@@ skipping 30 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "run tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]