revlog: compute snapshot depth on delta info We need the information to be available when choosing delta.

debugrevlog: display snapshot details per depth This help in understanding the final structure of build manifest. All data about snapshot (full and intermediate) are gathered into a sub-list for clarity. Since we do not produce such snapshots yet, the only thing changing in test output is the way the information is presented.

revlog: add a method to retrieve snapshot depth Some snapshot property (eg: maximum size) will depend on their depth.

debugrevlog: include information about intermediate snapshots As we are about to create intermediate snapshots, we need to have a way to debug them. We start by adding very simple debug output and more detailed output will comes in next changesets.