Generalize patch_project to patch root conversion.

recipe_modules/bot_update/example.expected/tryjob_v8.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_rel",
       "--slave",
       "totallyaslave-c4",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'src', 'url': 'svn://svn.chromium.org/chrome/trunk/src'}]",
       "--root",
       "src/v8",
       "--revision_mapping_file",
       "{\"src\": \"got_cr_revision\"}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--patch_url",
       "http://src.chromium.org/foo/bar",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--revision",

[Michael Achenbach, 2016/04/25 12:09:58]

Hmm, wonder if this should maybe not change? I didn't even know that feature (or
forgot about it in case I added it), but it looks like we can pass in the v8
revision to use as a property and if we do, it should be used instead of HEAD,
right?

[tandrii(chromium), 2016/04/25 13:37:52]

good point. Added a check to avoid ovewriting.

-      "src/v8@abc"
+      "src/v8@HEAD"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_TEXT@Some step text@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"fixed_revisions\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"src\": \"HEAD\", @@@",
-      "@@@STEP_LOG_LINE@json.output@    \"src/v8\": \"abc\"@@@",
+      "@@@STEP_LOG_LINE@json.output@    \"src/v8\": \"HEAD\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_failure\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_root\": \"src/v8\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_cr_revision\": \"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_cr_revision_cp\": \"refs/heads/master@{#170242}\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"src\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"step_text\": \"Some step text\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@got_cr_revision@\"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\"@@@",
       "@@@SET_BUILD_PROPERTY@got_cr_revision_cp@\"refs/heads/master@{#170242}\"@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]