configure jiri to manage a specific petal
usage: fx set-petal none|fuchsia|topaz "none" configures jiri to fetch all repos at the global integration version, and is the recommended default. Other values configure jiri to fetch that specific petal's repo at top-of-tree (HEAD) instead of the globally integration version. This means a "jiri update" will pull down committed but not yet integrated changes, which will be faster but may be less stable.