sync: Added logging of repo sync state and config options for analysis.

git_config.py:
+ Added SyncAnalysisState class, which saves the following data
  into the config object.
  ++ sys.argv, options, superproject's logging data.
  ++ repo.*, branch.* and remote.* parameters from config object.
  ++ current time as synctime.
  ++ Version number of the object.
+ All the keys for the above data are prepended with 'repo.syncstate.'
+ Added GetSyncAnalysisStateData and UpdateSyncAnalysisState methods
  to GitConfig object to save/get the above data.

git_trace2_event_log.py:
+ Added LogConfigEvents method with code from DefParamRepoEvents
  to log events.

sync.py:
+ superproject_logging_data is a dictionary that collects all the
  superproject data that is to be logged as trace2 event.
+ Sync at the end logs the previously saved syncstate.* parameters
  as previous_sync_state. Then it calls config's UpdateSyncAnalysisState
  to save and log all the current options, superproject logged data.

docs/internal-fs-layout.md:
+ Added doc string explaining [repo.syncstate ...] sections of
  .repo/manifests.git/config file.

test_git_config.py:
+ Added unit test for the new methods of GitConfig object.

Tested:
$ ./run_tests

$ repo_dev init --use-superproject -u https://android.googlesource.com/platform/manifest

Tested it by running the following command multiple times.
$ repo_dev sync -j 20
  repo sync has finished successfully

  Verified config file has [syncstate ...] data saved.

Bug: [google internal] b/188573450
Change-Id: I1f914ce50f3382111b72940ca56de7c41b53d460
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/313123
Tested-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Xin Li <delphij@google.com>

git_trace2_event_log.py

@@ -144,6 +144,19 @@ class EventLog(object):
     command_event['subcommands'] = subcommands
     self._log.append(command_event)
 
+  def LogConfigEvents(self, config, event_dict_name):
+    """Append a |event_dict_name| event for each config key in |config|.
+
+    Args:
+      config: Configuration dictionary.
+      event_dict_name: Name of the event dictionary for items to be logged under.
+    """
+    for param, value in config.items():
+      event = self._CreateEventDict(event_dict_name)
+      event['param'] = param
+      event['value'] = value
+      self._log.append(event)
+
   def DefParamRepoEvents(self, config):
     """Append a 'def_param' event for each repo.* config key to the current log.
 
@@ -152,12 +165,7 @@ class EventLog(object):
     """
     # Only output the repo.* config parameters.
     repo_config = {k: v for k, v in config.items() if k.startswith('repo.')}
-
-    for param, value in repo_config.items():
-      def_param_event = self._CreateEventDict('def_param')
-      def_param_event['param'] = param
-      def_param_event['value'] = value
-      self._log.append(def_param_event)
+    self.LogConfigEvents(repo_config, 'def_param')
 
   def ErrorEvent(self, msg, fmt):
     """Append a 'error' event to the current log."""