sync: use namedtuples for internal return values

Replace tuple returns with namedtuples, to simplify adding new fields.

Extend the Sync_NetworkHalf return value to:
 - success: True if successful (the former return value)
 - remote_fetched: True if we called `git fetch`

Change-Id: If63c24c2f849523f77fa19c05bbf23a5e9a20ba9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/344534


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>