When updating the fixVersion of an issue, the issue updater should use the same rules as the current hg updater:

     * If the main issue has the correct fixVersion, use it.
     * If an existing Backport has the correct fixVersion, use it.
     * If the main issue has a matching <N>-pool fixVersion, use it.
     * If an existing Backport has a matching <N>-pool fixVersion, use it.
     * If the main issue has a "scratch" fixVersion, use it.
     * If an existing Backport has a "scratch" fixVersion, use it.
     *
     * Otherwise, create a new Backport.
     *
     * A "scratch" fixVersion is empty, "tbd.*", or "unknown".