Nest multiple 'resolving hash xxx' steps under a single super step.

scripts/slave/recipes/bisect.expected/broken_good_revision_test.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/chromium/src.git'}, {'custom_deps': {'src/chrome/test/data/firefox2_profile/searchplugins': None, 'src/chrome/test/data/firefox2_searchplugins': None, 'src/chrome/test/data/firefox3_profile/searchplugins': None, 'src/chrome/test/data/firefox3_searchplugins': None, 'src/chrome/test/data/osdd': None, 'src/chrome/test/data/perf/frame_rate/private': None, 'src/chrome/test/data/plugin': None, 'src/chrome/test/data/ssl/certs': None, 'src/data/autodiscovery': None, 'src/data/esctf': None, 'src/data/mach_ports': None, 'src/data/memory_test': None, 'src/data/mozilla_js_tests': None, 'src/data/page_cycler': None, 'src/data/selenium_core': None, 'src/data/tab_switching': None, 'src/tools/grit/grit/test/data': None, 'src/webkit/data/bmp_decoder': None, 'src/webkit/data/ico_decoder': None, 'src/webkit/data/test_shell/plugins': None, 'src/webkit/data/xbm_decoder': None}, 'deps_file': '.DEPS.git', 'managed': True, 'name': 'src-internal', 'url': 'https://chrome-internal.googlesource.com/chrome/src-internal.git'}, {'deps_file': '.DEPS.git', 'managed': True, 'name': 'catapult', 'url': 'https://chromium.googlesource.com/external/github.com/catapult-project/catapult.git'}]",
       "--patch_root",
       "src",
@@ skipping 665 matching lines @@
       "git",
       "footers",
       "--position",
       "a6298e4afedbf2cd461755ea6f45b0ad64222222"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "resolving hash a6298e4afedbf2cd461755ea6f45b0ad64222222",
     "stdout": "/path/to/tmp/"
   },
   {
+    "cmd": [],
+    "name": "Resolving hashes"
+  },
+  {
     "cmd": [
       "git",
       "footers",
       "--position",
       "fc6dfc7ff5b1073408499478969261b826441144"
     ],
     "cwd": "[SLAVE_BUILD]/src",
-    "name": "resolving hash fc6dfc7ff5b1073408499478969261b826441144",
-    "stdout": "/path/to/tmp/"
+    "name": "Resolving hashes.resolving hash fc6dfc7ff5b1073408499478969261b826441144",
+    "stdout": "/path/to/tmp/",
+    "~followup_annotations": [
+      "@@@STEP_NEST_LEVEL@1@@@"
+    ]
   },
   {
     "cmd": [
       "git",
       "footers",
       "--position",
       "00316c9ddfb9d7b4e1ed2fff9fe6d964d2111111"
     ],
     "cwd": "[SLAVE_BUILD]/src",
-    "name": "resolving hash 00316c9ddfb9d7b4e1ed2fff9fe6d964d2111111",
-    "stdout": "/path/to/tmp/"
+    "name": "Resolving hashes.resolving hash 00316c9ddfb9d7b4e1ed2fff9fe6d964d2111111",
+    "stdout": "/path/to/tmp/",
+    "~followup_annotations": [
+      "@@@STEP_NEST_LEVEL@1@@@"
+    ]
   },
   {
     "cmd": [],
     "name": "Debug Info",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@Debug Info@bisector.lkgr: RevisionState(rev=chromium@e28dc0d49c, values=[], mean_value=None, std_dev=None)@@@",
       "@@@STEP_LOG_LINE@Debug Info@bisector.fkbr: RevisionState(rev=chromium@a6298e4afe, values=[19, 20, 21], mean_value=20.0, std_dev=1.0)@@@",
       "@@@STEP_LOG_LINE@Debug Info@@@@",
       "@@@STEP_LOG_LINE@Debug Info@Revision       Values         @@@",
       "@@@STEP_LOG_LINE@Debug Info@chromium@e28dc0d49c[]             @@@",
@@ skipping 18 matching lines @@
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]