rhg: desambiguate status without decompressing filelog if possible When status is unsure based on `stat()` and the dirstate if a file is clean or modified, we need to compare it against the filelog. This comparison can skip looking at contents if the lengths differ. This changeset optimize this further to deduce what we can about the length if the filelog without decompressing it or resolving deltas. Differential Revision: https://phab.mercurial-scm.org/D11965