cleanups in revlog.lookup
- add comments
- do a clean separation of the different cases
- don't use a list of each possible node when
doing the lookup, just keep the previous entry
% init
% commit
adding a
% annotate -c
8435f90966e4: a
% annotate -d
Thu Jan 01 00:00:01 1970 +0000: a
% annotate -n
0: a
% annotate -u
nobody: a
% annotate -cdnu
nobody 0 8435f90966e4 Thu Jan 01 00:00:01 1970 +0000: a