Avoid computing patch_root in get_files_affected_by_patch.

recipe_modules/tryserver/example.expected/with_rietveld_patch_new.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/apply_issue.py",
       "-r",
       "[SLAVE_BUILD]",
       "-i",
       "12853011",
       "-p",
       "1",
       "-s",
       "https://codereview.chromium.org",
       "--no-auth"
     ],
     "name": "apply_issue",
     "~followup_annotations": [
       "@@@STEP_LINK@Applied issue 12853011@https://codereview.chromium.org/12853011@@@"
     ]
   },
   {
     "cmd": [
       "git",
       "diff",
       "--cached",
       "--name-only"
     ],
-    "cwd": "[SLAVE_BUILD]",
+    "cwd": "[SLAVE_BUILD]/sub/project",
     "name": "git diff to analyze patch",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
-      "@@@STEP_LOG_LINE@files@foo.cc@@@",
+      "@@@STEP_LOG_LINE@files@sub/project/foo.cc@@@",
       "@@@STEP_LOG_END@files@@@",
       "@@@SET_BUILD_PROPERTY@failure_type@\"INVALID_TEST_RESULTS\"@@@",
       "@@@SET_BUILD_PROPERTY@subproject_tag@\"v8\"@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(1)"
     ],
     "name": "fail",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_TEXT@foo@@@",
       "@@@STEP_FAILURE@@@",
       "@@@SET_BUILD_PROPERTY@failure_hash@\"c2311ad770732eade3d2f48247abd147e40a70e7\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Step('fail') failed with return_code 1",
     "status_code": 1
   }
 ]