git recipe_module: add experimental git-cache support.

recipe_modules/git/example.expected/git-cache-checkout.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"
     ],
     "name": "git setup"
   },
   {
     "cmd": [
       "git",
       "retry",
+      "cache",
+      "fetch",
+      "-c",
+      "[GIT_CACHE]"
+    ],
+    "cwd": "[SLAVE_BUILD]/src",
+    "name": "fetch cache"
+  },
+  {
+    "cmd": [
+      "git",
+      "retry",
       "fetch",
       "origin",
       "master",
       "--recurse-submodules"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "git fetch"
   },
   {
     "cmd": [
@@ skipping 140 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "git bundle"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]