sync: Track last completed fetch/checkout

Save the latest time any project is fetched and checked out. This will
be used to detect partial checkouts.

Bug: b/286126621
Change-Id: I53b264dc70ba168d506076dbd693ef79a696b61d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/380514
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Joanna Wang <jojwang@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

main.py

@@ -427,7 +427,8 @@ class _Repo(object):
                 if not ok:
                     exception_name = type(e).__name__
                     git_trace2_event_log.ErrorEvent(
-                        f"RepoExitError:{exception_name}")
+                        f"RepoExitError:{exception_name}"
+                    )
                 raise
 
         try: