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>

subcmds/selfupdate.py

@@ -51,7 +51,7 @@ need to be performed by an end-user.
       _PostRepoUpgrade(self.manifest)
 
     else:
-      if not rp.Sync_NetworkHalf():
+      if not rp.Sync_NetworkHalf().success:
         print("error: can't update repo", file=sys.stderr)
         sys.exit(1)