log: move log file walk to its own function This moves the logic that determines which changesets to process during a 'hg log foo.txt' command. Putting it in its own function allows extensions to modify how the file log is traversed. For instance, the current implementation uses filelog revs heavily. Other implementations may not have filelog revs available. The function throws an exception if the traversal is not possible via the filelog, so the parent function can do things the slow way if necessary (by walking the entire commit history). Aside from the exception throwing, no logic is changed.