Make git.checkout return the commit hash.

recipe_modules/git/example.expected/count-objects_failed.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::git]/resources/git_setup.py",
       "--path",
       "[SLAVE_BUILD]/src",
       "--url",
       "https://chromium.googlesource.com/chromium/src.git"
@@ skipping 19 matching lines @@
       "checkout",
       "-f",
       "FETCH_HEAD"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "git checkout"
   },
   {
     "cmd": [
       "git",
+      "rev-parse",
+      "HEAD"
+    ],
+    "cwd": "[SLAVE_BUILD]/src",
+    "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]/src",
     "name": "git clean"
   },
   {
     "cmd": [
@@ skipping 110 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "git bundle"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]