sync: add retry to fetch operations

Add retries with exponential backoff and jitter to the fetch
operations. By default don't change behavior and enable
behind the new flag '--fetch-retries'.

Bug: https://crbug.com/1061473

Change-Id: I492710843985d00f81cbe3402dc56f2d21a45b35
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/261576
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: George Engelbrecht <engeg@google.com>