project: Do not exit early on --standalone-manifest.

After we successfully download the standalone manifest file, we cannot
exit early.

Bug: https://bugs.chromium.org/p/gerrit/issues/detail?id=15861
Change-Id: Ic47c9f7e9921851f94c6f24fd82b896eff524037
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/335974


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