Let config.sh take common |repo sync| options and pass them through.

For example, -d (check out the manifest revisions, instead of trying to
rebase the current HEADs onto them) is useful when changing branches or
devices, to prevent unwanted rebases that often fail.
Merge pull request #254 from jld/config-sync-flags

Let config.sh take common |repo sync| options and pass them through.