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>

tests/fixtures/test.gitconfig

@@ -11,3 +11,12 @@
 	intk = 10k
 	intm = 10m
 	intg = 10g
+[repo "syncstate.main"]
+	synctime = 2021-07-29T19:18:53.201328Z
+	version = 1
+[repo "syncstate.sys"]
+	argv = ['/usr/bin/pytest-3']
+[repo "syncstate.superproject"]
+	test = false
+[repo "syncstate.options"]
+	verbose = true