Fix file.copy recipe_module to not fail if files are already equal.

scripts/slave/recipes/skia/swarm_test.expected/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-MSAN.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/skia/tools/buildbot_spec.py",
       "/path/to/tmp/json",
       "Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-MSAN"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
@@ skipping 31 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  \"upload_dm_results\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"upload_perf_results\": false@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
-      "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
+      "\nimport shutil\nimport sys\n# Apparently, shutil.copy fails if files are binary-equal on *nix OSes.\n# See http://crbug.com/636168#c8.\nif hasattr(shutil, '_samefile'):\n  if shutil._samefile(sys.argv[1], sys.argv[2]):\n    sys.exit(0)\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/skia/infra/bots/assets/skp/VERSION",
       "/path/to/tmp/"
     ],
     "name": "Get downloaded SKP VERSION"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "42",
       "[SLAVE_BUILD]/tmp/SKP_VERSION"
     ],
     "name": "write SKP_VERSION"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
+      "\nimport shutil\nimport sys\n# Apparently, shutil.copy fails if files are binary-equal on *nix OSes.\n# See http://crbug.com/636168#c8.\nif hasattr(shutil, '_samefile'):\n  if shutil._samefile(sys.argv[1], sys.argv[2]):\n    sys.exit(0)\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/skia/infra/bots/assets/skimage/VERSION",
       "/path/to/tmp/"
     ],
     "name": "Get downloaded skimage VERSION"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
@@ skipping 58 matching lines @@
       "UBSAN_OPTIONS": "suppressions=[SLAVE_BUILD]/skia/tools/ubsan.supp"
     },
     "name": "dm"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]