obsolete: avoid using revset language to compute the obsolete revset This is part of a refactoring that moves some phase query optimization from revset.py to phases.py. See previous patches for the motivation. Now we have APIs in phasecache to get the non-public set efficiently, let's use it directly instead of going through the "not public()" revset language in "obsolete()" computation. This patch was meaured using: for i in 'public()' 'not public()' 'draft()' 'not draft()'; do hg perfrevset "$i"; hg perfrevset "$i" --hidden; done and no noticeable (> 1%) performance difference was observed.