Partial re-index from last known good state

ATM indexes break (by whatever circumstances) users need to perform a full re-index. Depending on the size off the repository this can take a long time.
If the user knows that the indexes were in a good state at a certain revision in the past then it would be very useful, if the user could trigger a "partial" re-index where only the content added after a certain revision was updated in the index.