sync: add multi-manifest support

With this change, partial syncs (sync with a project list) are again
supported.

If the updated manifest includes new sub manifests, download them
inheriting options from the parent manifestProject.

Change-Id: Id952f85df2e26d34e38b251973be26434443ff56
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334819


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