context: floor adjustlinkrev graph walk during copy tracing The `_adjustlinkrev` method gains an optional "stoprev" argument. The linkrev adjustment will give up once this floor is reached. The relevant functions using `_adjustlinkrev` are updated to pass an appropriate value in the copy tracing code. In some private repository, about 10% of the status call triggered the pathological case addressed by this change. The speedup varies from one call to another, the best-observed win is moving from 170s to 11s. The effect of this change can be seen in the public pypy repository, running the following command: hg perftracecopies --source 83c9ff0c0206 --destination 59c79103d5b0 before: 3.401753 seconds after: 2.634897 seconds (-23%)