format: introduce 'format.usegeneraldelta` This option will make repositories created as general delta by default but will not make Mercurial aggressively recompute deltas for all incoming bundle. Instead, the delta contained in the bundle will be used. This will allow us to start having general delta repositories created everywhere without triggering massive recomputation costs for all new clients cloning from old servers.

scmutil: extract general delta config handling in a function General delta is currently controlled by a single option, we will introduce a new one in the next changeset. We extract the logic in a function while it is simple.