sync: skip network half on repo upgrade

When repo upgrades itself, it will restart itself and rerun sync
command. At that point, we know that network half is already done and we
can just proceed with local half.

R=ddoman@google.com

Bug: b/377567091
Change-Id: I77205b1f2df19891597347d55283a617de3c6634
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/446201


Reviewed-by: Scott Lee <ddoman@google.com>
Tested-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>