git/housekeeping: Fix missing pack-objects strategy metric

Part of the metrics we expose in OptimizeRepository is the way we have packed objects. This is implemented via a switch over the strategy used to repack objects, which is fragile as it needs to be updated whenever we implement a new strategy. In fact, the switch is already missing the "geometric" strategy.

This MR refactors the strategy enumeration to be backed by a string so that we can easily adapt the metrics to just stringify the metric. Like this, it cannot become stale anymore.