sync: Show elapsed time for the longest syncing project

"Last synced: X" is printed only after a project finishes syncing.
Replace that with a message that shows the longest actively syncing
project.

Bug: https://crbug.com/gerrit/11293
Change-Id: I84c7873539d84999772cd554f426b44921521e85
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/372674
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Joanna Wang <jojwang@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

subcmds/checkout.py

@@ -58,7 +58,7 @@ The command is equivalent to:
                         success.append(project)
                     else:
                         err.append(project)
-                pm.update()
+                pm.update(msg="")
 
         self.ExecuteInParallel(
             opt.jobs,