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>